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Glued Langmuir-Blodgett Bilayers Having Unusually High He/CO2 Permeation Selectivities Junwei Li, Vaclav Janout, and Steven L. Regen* Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015 Received December 3, 2004 Single Langmuir-Blodgett bilayers derived from 5,11,17,23,29,35-hexakis[(N,N,N-trimethylammonium)N-methyl-37,38,39,40,41,42-hexakis-n-hexamedecyloxy-calix[6]arene hexachloride (1), which have been ionically cross-linked (i.e., “glued together”) with poly(acrylic acid), have been found to exhibit He/CO2 permeation selectivities as high as 150. This degree of selectivity, for a membrane that is less than 6 nm in thickness, is without precedent. In principle, materials of this type could lead the way to improved membranes for hydrogen purification.
This paper records our finding of unusually high He/ CO2 selectivity by ionically cross-linked (i.e., “glued”) Langmuir-Blodgett (LB) bilayers. Specifically, we report that single LB bilayers composed of calix[6]arene 1 and poly(acrylic acid) (PAA) exhibit He/CO2 selectivities as high as ca. 150 [Chart 1]. This degree of selectivity, for a membrane that is less than 6 nm in thickness, is without precedent. In principle, materials of this type could lead the way to improved membranes for hydrogen purification. The challenges that lie ahead in trying to move toward a hydrogen economy are substantial.1-3 At present, the only practical means for producing hydrogen is based on “steam reforming” processes, whereby natural gas and steam are converted into H2 and CO2.4 While H2 and CO2 can be readily separated by adsorption and cryogenic methods, permeation through semipermeable membranes offers the greatest opportunity for developing highly energy-efficient separation processes. The challenge to the polymer chemist is to create new materials having high H2/CO2 permeation selectivities and high flux.5 The permeation of a gas across a nonporous organic polymer membrane is governed by a solution-diffusion mechanism, in which its permeability coefficient, P, is the product of a solubility coefficient (S) and a diffusivity coefficient (D); that is, P ) SD.5 Although the solubility term plays a minor role for the permeation of H2, it is often the dominant parameter for CO2. For this reason, most H2/CO2 selectivities that have been recorded are low, typically lying between 0.1 and 10. In essence, the lower diffusivity that is associated with the larger CO2 molecule is compensated for by a higher solubility in the organic membrane. Recently, we reported the synthesis of glued LB bilayers derived from 1, which have high He/N2 permeation selectivities.6 Given the porous structure of 1, it occurred to us that such membranes might also show high He/CO2 * To whom correspondence should be addressed. E-mail: slr0@ lehigh.edu. (1) Pacala, S.; Socolow, R. Science 2004, 305, 968-972. (2) Turner, J. A. Science 2004, 305, 972-974. (3) Demirdoven, M.; Deutch, J. Science 2004, 305, 974-976. (4) Dicks, A. L. J. Power Sources 1996, 61, 113-124. (5) For a review of organic membranes and transport mechanisms for gas permeation, see: (a) Stern, S. A. J. Membr. Sci. 1994, 94, 1-65. (b) Koros, W. J.; Fleming, G. K. J. Membr. Sci. 1993, 83, 1-80. (c) Koros, W. J. Chem. Eng. Prog. 1995, 91, 68-81. (d) Robeson, L. M.; Burgoyne, W. F.; Langsam, M.; Savoca, A. C.; Tien, C. F. Polymer 1994, 35, 4970-4978. (e) Polymers For Gas Separations; Toshima, N., Ed.; VCH: New York, 1992. (f) Robeson, L. M. J. Membr. Sci. 1991, 62, 165-185. (g) Robeson, L. M. J. Membr. Sci. 1991, 62, 165-185. (h) Freeman, B. D. Macromolecules 1999, 32, 375-380.
Chart 1
selectivities. Specifically, we reasoned that if molecular sieving contributes to the overall permeation rates, then high He/CO2 should be possible since molecular sieving is determined solely by the size of the permeants relative to the size of the molecular pores. In other words, CO2 solubility would be of lesser importance. With this rationale in mind, we prepared a series of glued LB bilayers from 1 and PAA and measured their permeability with respect to He, CO2, O2, and N2. For this purpose, bilayers were fabricated using a subphase that had a pH ranging from 10.0 to 3.5. As we have shown previously, gluing efficiency and film quality increase as the pH of the subphase that is used for the LB transfers decreases.6c Given the kinetic diameters for He, CO2, O2, and N2 of 0.260, 0.330, 0.346, and 0.363 nm, respectively, diffusion-controlled permeation is expected to result in the following relative permeabilities: He > CO2 > O2 > N2.7 Helium was chosen as a surrogate for hydrogen because it avoids the hazards in working with a combustible permeant. We also briefly examined glued bilayers made from a nonporous analogue of 1 (i.e., 2).6d (6) (a) Yan; X.; Janout, V.; Hsu, J. T.; Regen, S. L. J. Am. Chem. Soc. 2003, 125, 8094-8095. (b) Li, J.; Janout, V.; Regen, S. L. Langmuir 2004, 20, 2048-2049. (c) Li, J.; Janout, V.; McCullough, D. H., III; Hsu, J. T.; Troung, Q.; Wilusz, E.; Regen, S. L. Langmuir 2004, 20, 82148219. (d) McCullough, D. H., III; Janout, V.; Li, J.; Hsu, J. T.; Troung, Q.; Wilusz, E.; Regen, S. L. J. Am. Chem. Soc. 2004, 126, 9916-9917. (7) Breck, D. W. Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley: New York, 1973; p 626.
10.1021/la047020p CCC: $30.25 © 2005 American Chemical Society Published on Web 02/03/2005
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Table 1. Permeability across Glued LB Bilayersa 106 P/l (cm3/cm2-s-cmHg) bilayer
pH
He
CO2
O2
N2
RHe/N2
RHe/CO2
1
10.0 6.0 5.2 4.4 3.5 4.4 3.5
264b 232b 178b 71b 60 180 91
47 73 26 1.1 0.39 76 11
16 11 3.8 0.54 0.27 12 2.0
2.5b 1.5b 0.57b 600 95 380
5.5 3.2 6.8 65 150 2.4 8.3
2
a All LB films were fabricated using a constant surface pressure of 30 dyn/cm (25 °C) and a 0.1 mM repeat unit concentration of PAA in the subphase (MW 240 000). Normalized flux values (P/l) were calculated by dividing the observed flux by the area of the membrane (9.36 cm2) and the pressure gradient (10 psi) employed. Here, l is the thickness of the composite membrane. Each set of data corresponds to a separate composite membrane made using a ca. 15-µm-thick PTMSP support plus a single glued bilayer of 1. All measurements were made at ambient temperatures. Values reported are averages of 5-10 measurements for each sample. b Taken from ref 6c.
Figure 2. Comparison of He/CO2 selectivities versus the permeability coefficient, P, for He for a broad range of organic polymers. The open circle represents the permeability properties of a glued bilayer derived from 1 and PAA (pH 3.5).
series-resistance model), then the intrinsic permeability of the LB film can be calculated from eq 1 and the ellipsometric film thickness of the bilayer (5.6 nm).6c,8 Here, kcomp, ksup, and kLB are the normalized flux for the composite, the PTMSP support, and the LB film, respectively. Thus, for a glued bilayer made from 1 and PAA (pH 3.5), having a He/CO2 selectivity of 150, the calculated permeability coefficient for He is 0.4 Barrer (1 Barrer ) 1 × 10-10 (cm3‚cm/cm2-s-cm Hg).
1/kcomp ) 1/ksup + 1/kLB
Figure 1. Plot of normalized flux versus kinetic diameter for the permeation of He, CO2, O2, and N2 across bilayers derived from 1 and PAA using a subphase pH of 10 (b), 6.0 (9), 5.2 (2), 4.4 (O), and 3.5 (0).
Experimental procedures that were used in synthesizing 1 and 2, depositing glued bilayers onto poly[1-(trimethylsilyl)-1-propyne] (PTMSP) supports, and measuring their barrier properties were similar to those previously described.6c Table 1 summarizes our main findings. For all of the membranes examined, the relative permeabilities (expressed as normalized flux) were He > CO2 > O2 > N2. In addition, the use of a lower pH in fabricating glued bilayers of 1 resulted in higher He/CO2 selectivities as well as higher He/N2 selectivities. A plot of normalized flux versus kinetic diameter clearly shows this pH dependency, and also the dominating effect that the size of the permeant has on its permeability across these membranes (Figure 1). Glued bilayers of 2 that were fabricated using a pH of 4.4 and 3.5 showed moderate He/N2 selectivities but relatively low He/CO2 selectivities. However, whereas 1 gave transfer ratios for the downand up-trips of 1.0 ( 0.1, the transfer ratios for 2 for the down- and up-trips were 0.8 ( 0.1 and 0.7 ( 0.1, respectively, at pH 4.4; at pH 3.5, they were 0.4 ( 0.1 and 0.8 ( 0.1, respectively, Thus, permeation through defects may be contributing to the lower selectivity and higher permeability of these glued bilayers. If it is assumed that the resistance of a glued bilayer/ PTMSP composite is equal to the sum of the resistances of the LB bilayer and that of the PTMSP support (i.e., the
(1)
To place these permeation properties into perspective, a “trade-off” plot for He/CO2 selectivity versus helium permeability is shown in Figure 2 for a series of representative polymers that have been reported in the literature. Also included is a solid line that corresponds to a theoretical “upper bound”, that is, the maximum selectivity and permeability that is predicted based on a solution-diffusion transport mechanism.5h To our knowledge, the only polymer that has a He/CO2 selectivity that is as high as that of a glued bilayer made from 1 and PAA is poly(acrylonitrile).9 Both of these membranes lie close to the upper bound, and both have a He/CO2 selectivity that is in excess of 100. A key difference between the two materials, however, is that whereas the glued bilayer can be fabricated as defect-free, 5.6-nm-thick membranes, poly(acrylonitrile) cannot. The fact that the relative permeabilities across glued bilayers derived from 1 and PAA follow He > CO2 > O2 > N2 clearly indicates that solubility contributions play a minor role in these systems. The present data, however, do not allow us to quantify the extent to which molecular sieving may be operating. Nonetheless, it is clear that the combination of unusually high He/CO2 permeation selectivity and extreme thinness makes these glued bilayers unique and promising materials for hydrogen purification. Studies that are currently in progress are aimed minimizing solution-diffusion and maximizing molecular sieving pathways (i.e., crossing the upper bound) by rational design. The results of these efforts will be reported in due course. (8) (a) Rose, G. D.; Quinn, J. A. Science 1968, 159, 636-637. (b) Rose, G. D.; Quinn, J. A. J. Colloid Interface Sci. 1968, 27, 193-207. (9) Allen, S. M.; Fujii, M.; Stannett, V.; Hopfenberg, H. B.; Williams, J. L. J. Membr. Sci. 1977, 2, 153-163.
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Acknowledgment. This material is based upon work supported by the U.S. Army Natick Soldier Center, Natick, MA, under Contract No. DAAD16-02-C-0051. We are grateful to Professor Benny Freeman (University of Texas) for valuable discussions.
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Supporting Information Available: Literature references to the permeation data shown in Figure 2. This material is available free of charge via the Internet at http://pubs.acs.org. LA047020P
