Facilitated Oxygen Transport Membranes of Picket-fence

5954

Ind. Eng. Chem. Res. 2003, 42, 5954-5958

Facilitated Oxygen Transport Membranes of Picket-fence Cobaltporphyrin Complexed with Various Polymer Matrixes Baoqing Shentu†,‡ and Hiroyuki Nishide*,† Department of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan, and Institute of Polymer Engineering, Zhejiang University, Hangzhou 310027, China

A series of membranes having a selective and reversible oxygen-binding capacity were prepared by complexing picket-fence cobaltporphyrin (CoP) as an oxygen carrier with various polymer matrixes. The oxygen-absorption isotherm and oxygen permeation in and through the membranes were properly analyzed by dual-mode models to give the physical solubility coefficient of oxygen (kD), the physical diffusion coefficient of oxygen through the membrane (DD), and the apparent diffusion coefficient of oxygen for hopping between fixed carriers (DC). The oxygen-binding equilibrium constant (K) of CoP was independent of the polymer matrix species. The DC value was smaller than the DD value, but the DC/DD ratio still remained in the range of 1/5-1/50, indicating that CoP fixed in the membrane reversibly binds oxygen and facilitates oxygen transport. 1. Introduction Oxygen and nitrogen are among the top five largest commodity chemicals in major countries.1 They are usually produced from air by the cryogenic distillation process and pressure swing adsorption method. A polymer membrane process has been considered to be an intriguing candidate as an energy-saving alternative.2,3 For gas separation, polymers with both high permeability and permselectivity are desirable. However, permeability through a polymer membrane inversely correlates with the permselectivity of oxygen relative to nitrogen, and until now, there have been no reports that simple polymer membranes have both high permeability and permselectivity.4-6 A solid membrane using the concept of facilitated transport mediated with a fixed carrier has thus been considered to be a very promising alternative. During facilitated transport, the transport of a specific permeate occurs by being mediated by a fixed carrier in the solid membrane, in addition to the normal physical permeation route, because the carrier in the membrane can reversibly bind the specific permeate.6-8 Therefore, both the permeability and permselectivity can be remarkably improved. For example, silver ions reversibly and specifically react with olefins and, thus, act as olefin carriers.9,10 The membranes prepared from silver-polymer complexes could separate unsaturated hydrocarbons (olefins) from saturated ones (paraffins). The authors have been studying metalloporphyrins as efficient oxygen carriers and applying them as oxygen absorbents,11,12 optical oxygen sensors,13,14 and oxygenpermselective membranes.15-20 A typical example of a metalloporphyrin is meso-R,R,R,R-tetrakis(o-pivalamidophenyl)porphyrinatocobalt(II)21,22 (CoP, Chart 1), which has four pivalamido groups on one side of the porphyrin plane to provide a cavity for oxygen binding while the * To whom correspondence should be addressed. E-mail: [email protected]. † Waseda University. ‡ Zhejiang University.

Chart 1. Picket-Fence Cobaltporphyrin Complexed with the Imidazole Residue of Polymer Matrixes

other side remains available for complexing with an imidazole ligand, such as the imidazole residue of the 1-vinylimidazole polymers, to improve the oxygenbinding affinity. We have demonstrated facilitated and selective oxygen transport through solid polymer membranes of CoP.15-19 The specific and reversible oxygenbinding reaction of CoP establishes a concentration gradient of oxygen from the upstream side to the downstream side in the membrane and yields facilitated oxygen transport relative to nitrogen.2,7 In addition to the chemical reactivity of CoP, the chemical structure of the polymer matrix has to be carefully designed to obtain a solid CoP membrane that

10.1021/ie020770e CCC: $25.00 © 2003 American Chemical Society Published on Web 10/01/2003

Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 5955

is easily applicable to air separation. First, the polymer matrix is expected to complex with the fifth coordinative site of CoP to improve the oxygen-binding affinity of CoP and to be homogeneously held and fixed with CoP in the membrane. From this viewpoint, the copolymers of vinylimidazole satisfy this requirement.7,12 Second, the polymer matrix should have a good membrane-forming ability. Additionally, the polymer matrixes are expected to provide a microenvironment surrounding CoP that is likely to contribute to the facilitated transport. That is, issues on the effects of the polymer matrix species on facilitated oxygen transport remained to be studied. (i) The oxygen-binding affinity of the fixed carrier is an important factor in increasing facilitated oxygen transport.22 The effect of the polymer matrixes on the oxygen-binding affinity of CoP needs to be clarified. (ii) In contrast to the clear effect of the selective and enhanced solubility of oxygen caused by the fixed carrier, the oxygen diffusivity in the CoP membranes fixed in different polymer matrixes has not been precisely analyzed. (iii) The balance of the facilitated transport efficiency, permeability, and permselectivity also can be addressed for the different matrix membranes. Four kinds of polymers having different structures were chosen as the membrane matrixes for the described application. Alkyl methacrylate copolymers were used as membrane matrixes because of their toughness and good membrane-forming ability.23,24 Poly(vinylidene dichloride) copolymer and poly(1-trimethylsilyl-1-propyne) were selected as the gas barrier and the highest gas permeable polymer, respectively.25-28 The present paper reports the facilitated transport of oxygen in membranes prepared by complexing CoP with four polymer matrixes. An electrochemical method24 having a high sensitivity was used to measure the oxygen permeability through the membrane. The oxygen absorption isotherm and permeation through the CoP membrane were analyzed by dual-mode models. Factors that realize the efficiently facilitated transport of oxygen are also discussed. 2. Experimental Section 2.1. Materials. Meso-R,R,R,R-tetrakis(o-pivalamidophenyl)porphyrinatocobalt(II) (CoP) was synthesized as described in the literature.21 The cobalt(III) derivative of CoP was prepared by oxidizing CoP with nitrogen monoxide. Poly(octyl methacrylate-co -vinylimidazole) and poly(lauryl methacrylate-co-vinylimidazole) (OIm and LIm, respectively) were prepared by radical copolymerization using azobis(isobutyronitrile) as the radical initiator. Poly(vinylidene dichloride-co-vinylimidazoleco-methyl methacrylate) (CIm) was obtained by emulsion copolymerization initiated by ammonium persulfate and sodium thiosulfate. Poly(1-trimethylsilyl-1-propyne) (SP) was prepared in the presence of TaCl5 as the catalyst, according to a previously reported method.27 The imidazole residue contents and the molecular weights of the copolymers were determined to be 45 mol % and 1.3 × 105 for OIm, 52 mol % and 1.6 × 105 for LIm, 36 mol % and 3.0 × 105 for CIm, and 0 mol % and 8.0 × 105 for SP, by elemental analysis and GPC, respectively. 2.2. Membrane Preparation. Dichloromethane solutions of OIm or LIm and CoP (molar ratio [imidazolyl residue]/[CoP] ) 29/1) and chloroform solutions of CIm

Figure 1. Oxygen-binding equilibrium curves of the CoP fixed in the four polymer matrixes. Oxygen binding/% means the percentage of CoP that binds with oxygen in the membrane under the given oxygen partial pressure (pO2). Inset: Visible absorption spectral change with oxygen partial pressure from 0 to 76 cmHg.

and CoP (molar ratio [imidazolyl residue]/[CoP] ) 36/ 1) were mixed to complex the imidazole residue of the copolymer with the fifth coordination site of CoP (Chart 1) under a nitrogen atmosphere. Because of the lack of an imidazole residue in poly(trimethylsilylpropyne) (SP), in contrast to the other three matrix polymers, a chloroform solution of SP and benzylimidazole (BIm), as the low-molecular-weight ligand, was mixed with that of CoP (molar ratio [imidazole]/[CoP] ) 5/1) under nitrogen. The mixed solution was carefully cast on a Teflon plate under an oxygen-free atmosphere, after which it was dried in vacuo, to produce a red-colored membrane with a thickness of ca. 30 µm. 2.3. Spectroscopic, Absorption, and Permeation Measurements. Oxygen binding to CoP was measured by comparing the spectral difference in the UV-visible absorption using a UV spectrophotometer (Shimadzu model UV-2100). The amounts of oxygen and nitrogen absorbed into the membrane were measured by the pressure decrease in the constant-volume chamber using a Baratron absolute pressure gauge (MSK Instruments). The apparatus consisted of a vacuum line mounted in a thermocontrolled air bath. Oxygen permeation through the membranes was electrochemically measured by detecting the reduction current of permeated oxygen on a carbon electrode at 25 °C.24 3. Results and Discussion 3.1. Chemically Specific Oxygen Binding and Absorption. Complexation of CoP to an imidazole residue of the 1-vinylimidaozle copolymers or benzylimidazole yielded both an active and homogeneous dispersed CoP for oxygen binding in the solid membrane state. The membrane was homogeneously and deeply red-colored with CoP but was still transparent. X-ray diffractometry of the membrane gave no significant pattern, which indicated that the polymer membrane was in an amorphous state and verified that the CoP had not crystallized out. The red color of the transparent CoP membranes indicated that the molecules reversibly changed from the deoxy form (λmax ) 530 nm) to the oxy or oxygenbinding form (λmax ) 548 nm) with an isosbestic point at 538 nm, in response to the partial oxygen pressure of the atmosphere (inset of Figure 1). This spectral change was attributed to the rapid and reversible oxygen-adduct formation of the CoP complex even in the solid membrane (eq 1) and was monitored by UV-

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Figure 2. Absorption isotherms of oxygen and nitrogen for the CoP-OIm membranes at 25 °C. Table 1. Glass Transition Temperatures (Tg) and Physical Oxygen Solubility Coefficients (kD) of the CoP Polymer Membranes and Oxygen-Binding Equilibrium Constants (K) of CoP

membranea

Tg (°C)

102 kD [cm3 (STP) cm-3 cmHg-1]

102 Kapp (cmHg-1)

K (mol-1 L)

OIm LIm CIm SP/BIm

37 26 47 200

5.8 7.0 1.2 12

3.6 5.6 0.85 8.0

14 18 16 15

a

CO2 ) kDpO2 +

CoP content ) 10 wt %.

visible absorption spectrometry. K

Im-CoP + O2 y\z Im-CoP-O2

(1)

K ) [Im-CoP-O2]/[Im-CoP][O2]

(2)

Kapp ) 1/p50 ) [Im-CoP-O2]/[Im-CoP][atmospheric O2] (3) K ) Kapp/kD

The amounts of oxygen and nitrogen taken up by the membranes were measured using an absorption apparatus to give the absorption isotherms of oxygen and nitrogen (Figure 2). The oxygen absorption amount was enhanced for all of the CoP membranes and increased with the CoP content. The oxygen absorption obeyed a Langmuir-type isotherm, whereas the nitrogen absorption showed a proportional increase with nitrogen pressure according to Henry’s law. These results indicate that the CoP fixed in the membrane acts as a chemically specific and reversible oxygen-binding site. At low oxygen pressure, the oxygen absorption amounts for the CoP membranes complexed with different polymer matrixes were almost the same, whereas the absorption amounts were quite different at high pressure for the same CoP concentration. This means that, at a low oxygen pressure, the oxygen amount absorbed by the membrane mainly resulted from chemical absorption and was almost the same for the different matrix membranes. In contrast, with increasing oxygen pressure, physical absorption gradually became predominant, and the total absorption amount was different because of the different physical oxygen solubility. The absorption amount of oxygen can be analyzed using a dual-mode absorption model,29 represented by the following equation

(4)

Here, Im represents the imidazole residue of the polymer, [O2] (mol L-1) is the oxygen concentration (surrounding CoP) in a membrane, K in eq 2 is the oxygenbinding equilibrium constant (L mol-1), and kD is the physical oxygen solubility coefficient in the membrane [cm3(STP) cm-3 cmHg-1] (estimated in this study by the absorption measurement given in Figure 2). The oxygen-binding equilibrium curve, i.e., the oxygenbinding percentage vs the atmospheric partial oxygen pressure, obeyed the Langmuir-type isotherm for the CoP membrane to give the oxygen-binding affinity [p50 (cmHg), the atmospheric oxygen partial pressure at which one-half of the CoP binds with oxygen] or the apparent oxygen-binding equilibrium constant, Kapp (cmHg-1), as in eq 3. The oxygen-binding equilibrium constant, K (mol-1L), can be derived from the reduction of Kapp by kD as shown in eq 4. The Kapp and K values are summarized in Table 1 for the four CoP membranes; the reduced K values for oxygen binding to CoP fixed in the membranes remained constant, indicating that the oxygen-binding nature of the CoP was not directly affected by the matrix species, or that the oxygenbinding equilibrium of the CoP complex itself is essentially independent of the surrounding polymer.

CCKapppO2 1 + KapppO2

(5)

Here, CO2 is the absorption amount of oxygen, pO2 is the partial oxygen pressure, and CC is the saturated amount of oxygen specifically chemically absorbed to the fixed carrier. The oxygen absorption isotherm (pO2 vs CO2) was analyzed using eq 5, i.e., CO2 was plotted versus 1/(1 + KapppO2). CC had almost the same value [1.1, 0.7, and 0.8 cm3(STP) cm-3] for the OIm-, LIm-, and CImCoP membranes, respectively. The kD value was in the order SP > LIm > OIm > CIm: SP gave a large kD in comparison with the other polymer matrixes, which could be explained by the extraordinarily large free volume of SP.28 For LIm and OIm, the order of kD coincided with the order of the free volume of the polymer matrixes, which was estimated by the glass transition temperature of the copolymers. For the CImCoP membrane, the kD value was significantly low because of its densely interacting interpolymer structure of the vinylidene dichloride residue. 3.2. Oxygen Permeation through the Cobaltporphyrin Membranes. Oxygen permeation through the membranes was electrochemically measured by detecting the reduction current of the permeated oxygen.24 This method is applicable to dilute oxygen, i.e., to permeation measurements at the lower upstream oxygen pressure, with appropriate sensitivity because the permeated oxygen is completely consumed by the electrochemical reduction on the downstream side. In addition, no mechanical pressure was applied to the membrane, therefore, making possible the measurement of the brittle membrane. Figure 3 shows the effect of the upstream oxygen pressures (pO2) on the oxygen permeability coefficients (PO2) of the OIm-CoP membranes. PO2 increased steeply with decreasing pO2. On the other hand, PO2 for the control membrane composed of inactive (oxidized) CoIIIP was small and independent of pO2. These results indicate that the CoP carrier fixed in the membrane interacts with oxygen and facilitates oxygen transport.

Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 5957 Table 2. Oxygen Permeability Coefficients (PO2), Facilitation Factors (F), Physical Diffusion Coefficients (DD), and Apparent Diffusion Coefficients between the Fixed Carriers (DC) for the CoP Membranes membranea

109 PO2b

F

108 DDc

108 DCc

DC/DD

OIm LIm CIm SP/BIm

5.8 12 0.18 610

9 3 18 1.1

2.7 6.7 0.083 830

0.22 0.30 0.015 15

0.08 0.04 0.18 0.02

a CoP content)10 wt %. b In cm3 (STP) cm cm-2 s-1 cmHg-1 at pO2 ) 0.5 cmHg. c In cm2 s-1.

Figure 3. Oxygen permeability coefficient (PO2) at various upstream oxygen pressure (pO2) for the OIm-CoP membrane. CoP content: (b) 10 wt %, (9) 20 wt %, (O) inactive CoIIIP 10 wt %. Inset: Facilitation factor vs CoP concentration in the membrane.

Figure 5. Dual-mode transport model.

Figure 4. Effect of polymer matrixes on oxygen permeability coefficients (PO2) for CoP (10 wt %) membranes at various upstream oxygen pressures (pO2).

Figure 4 shows the effect of the polymer matrixes on PO2 for the CoP membranes at various pO2 values. The PO2 values were in the order SP/BIm > LIm > OIm > CIm. The largest PO2 for the SP-CoP membrane could be explained by the larger solubility and diffusivity of oxygen in SP. The low PO2 for the CIm-CoP membrane could be ascribed to the suppressed diffusivity and solubility of oxygen in CIm. For the alkyl methacrylate copolymers, the higher oxygen permeability of the LImCoP membrane compared to OIm-CoP is explained by its lower Tg value. The facilitation factor (F, defined in this paper as the ratio of PO2 at 0.5 cmHg to that at 76 cmHg) was largest for CIm and the smallest for SP (Table 2). This means that the membranes with a high PO2 usually resulted

in a low F. The oxygen permeability coefficient for the CIm-CoP membrane was around 10-10 cm3(STP) cm cm-2 s-1 cmHg-1 and small in comparison with the values for the other membranes; however, it should be noted that the F value for CIm-CoP was the highest among the membranes and the PO2 enhancement for a low upstream oxygen pressure was significant (Figure 4). The inset of Figure 3 indicates that F increases with the CoP carrier content of the membrane in this content range. 3.3. Analysis of the Facilitated Oxygen Transport. Figure 5 schematically represents the model for oxygen permeation in the fixed carrier membrane that is governed by both the Henry and Langmuir modes.6,8 Oxygen physically dissolves in and diffuses through the membrane via the upper permeation route. In addition, the lower permeation route shows that oxygen is specifically and chemically taken up by a selective binding reaction to the CoP fixed in the membrane and diffuses through the fixed carrier by repeating the binding and releasing reactions to and from CoP. The oxygen transport is accelerated by this carrier-mediated mode in addition to the physical mode. This dual-mode model is mathematically given by

PO2 ) kDDD +

CCKappDC 1 + KapppO2

(6)

Here, DD is the physical diffusion coefficient of oxygen through the membrane, and DC is the apparent diffusion coefficient of oxygen for hopping between the fixed carriers. The PO2 vs pO2 data were analyzed using eq 6, that is, PO2 was plotted versus 1/(1 + KapppO2). DD was calculated from the permeability coefficient of the membrane containing 10 wt % CoIIIP. The plots gave a linear relationship, and DC was obtained from the slope given in Table 2. For membranes with the same CoP concentration, DD for SP was the largest, followed, in order,

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by those of LIm, OIm, and CIm. This result suggests that DD is proportional to the free volume of the matrix. DC also varies with the polymer matrix species in a similar manner. Although DC represents the apparent diffusion coefficient for oxygen hopping between fixed carriers, it was still affected by the oxygen solubility (kD) and free volume of the matrix. DC was smaller than DD because the chemical binding reaction of oxygen with CoP suppresses the diffusivity of oxygen. Although the value of DC was small, its ratio to DD was 1/5-1/50, indicating that oxygen-binding to CoP is reversible and that oxygen transport is effectively facilitated by the fixed carrier. 4. Conclusion Membranes having a selective and reversible oxygenbinding capability were prepared by complexing the picket-fence cobaltporphyrin (CoP) with four polymer matrixes. The CoP carrier fixed in the membranes interacted with oxygen and facilitated oxygen transport. Although the SP/BIm-CoP membrane produced the highest oxygen permeability, its separation and facilitation factor remained the lowest. Despite the low oxygen permeability of the CIm-CoP membrane, it exhibited the highest facilitation and separation factor, which could be applicable for air separation if an appropriate thin membrane could be prepared. Acknowledgment This work was partially supported by a Grant-in-Aid for Scientific Research (No. 13031072) from MEXT, Japan. B. S. expresses her thanks for a Monbusho Scholarship from MEXT, Japan. Literature Cited (1) Chem. Eng. News 2002, June 24. (2) Baker, W. R. Future Directions of Membrane Gas Separation Technology. Ind. Eng. Chem. Res. 2002, 41, 1393. (3) Drioli, E.; Romano, M.; Progress and New Perspective on Integrated Membrane Operations for Sustainable Industrial Growth. Ind. Eng. Chem. Res. 2001, 40, 1277. (4) Freeman, B. D. Basis of Permeability/Selectivity Tradeoff Relations in Polymeric Gas Separation Membrane. Macromolecules 1999, 32, 375. (5) Koros, W. J.; Mahajam, R. Pushing the Limits on Possibilities for Large Scale Gas Separation: Which Strategies. J. Membr. Sci. 2000, 175, 181. (6) Nishide, H.; Tsuchida, E. Polymer Complex Membranes for Gas Separation. In Polymer for Gas Separation; Toshima, N., Ed.; VCH Publishers: New York, 1992; pp 183-219. (7) Way, J. D.; Noble, R. D. Facilitated Transport. In Membrane Handbook; Sirkar, K. K., Ho, W. S., Eds.; Van Nostrand Reinhold: New York, 1993; pp 833-866. (8) Nishide, H.; Tsuchida, E. Transport Phenomena and Separation of Small Molecules. In Macromolecules-Metal Complexes; Ciardelli, F., Tsuchida, E., Woehrle, D., Eds.; Springer-Verlag: Berlin, 1996; pp 175-190. (9) Ho, W. S. Polymeric Membrane and Process for Separating Aliphatically Unsaturated Hydrocarbons. U.S. Patent 5,015,268, 1991. (10) Pinnau, I.; Toy L. G.; Casillas, C. Olefin Separation Membrane and Process. U.S. Patent 5,670,051, 1999. (11) Nishide, H.; Chen, X.-S.; Tsuchida, E. Oxygen-Carrying and Oxygen-Permeating Polymers. In Functional Monomers and

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Received for review September 26, 2002 Revised manuscript received June 26, 2003 Accepted July 27, 2003 IE020770E