(77h) Nanoporous Metal Soap Films Prepared By Interfacial Polymerization and Melt Processing for Gas Separation

Metal soaps, or metal alkanoates, as a kind of liquid crystal when heating, are constructed by metal cations and organic molecules composed of a variety of functional groups linked to hydrocarbon chains.^1-4 They have a bi-layered arrangement with a layer made of metal cations linked with the functional groups and another layer made of lipidic hydrocarbon chains. The metal cations are linked by functional groups, forming a layer of rods/chains and sheets, while the functional groups are co-bonded to hydrocarbon chains, which act as the other layers (Figure 1).

Due to the high flexibility of long alkyl chains in the structures, most metal soaps can melt at a certain temperature, forming a disordered/amorphous liquid phase or a mesophase. The liquid phase can be readily quenched by cooling down upon which the disordered structures quickly turn back to the original crystalline state. Because of the hydrophobicity of the long alkyl chains, metal soaps have been used as "green" corrosion barrier of metals by constructing metal soaps on the surface of metals. In addition, due to the highly polarization between metal cations and lipidic chains during melting, metal soaps have been proved to be potentially used as nonlinear-optical materials.⁴

To the best of our knowledge, there is no literatures reporting on metal soaps based membranes, to date. This presents an opportunity to evaluate this class of material for gas separation membranes especially in the context of large number of possible ordered porous metal soap which could allow separation of a number of gas pairs by size sieving. Additionally, metal soap provide processability advances as one could synthesize membranes by using solid to liquid transition ability of metal soaps, making such membranes easy to scale-up.

In this presentation, firstly, I will introduce two facile methods, interfacial crystallization and melting method, to synthesize membranes which constructed by three metal soaps, CaC₁₂ (Ca(SO₄C₁₂H₂₅)₂), ZnC₇ (Zn(COOC₆H₁₃)₂) and CuC₁₀ (Cu(COOC₉H₁₉)₂). Regarding to melting method, the membrane formed by heating the powders of these structures in low temperatures (120 -150 ËšC) in less than 1 hour (Figure 1A). While as to interfacial crystallization, the continuous film is formed on the whole area of water surface in few seconds (which we found to be a facile way to synthesize film with very low thickness). And the film could be easily transported by scooping with any substrate (Figure 1B). Both methods show potential of easy-processability and facile scale-up. Secondly, I will discuss the crystal structures and characterizations of these metal soaps. All of them have pores coming from the gaps around 3 Ã…, which means they could be used in hydrogen sieving (H₂, 2.89 Ã…) or carbon capture (CO₂, 3.3 Ã…) (Figure 1C-1E). Scanning electronic microscope (SEM) images showed the thickness of these membranes are less than 5 μm (Figure 1F-1H). X-ray diffraction (XRD) data confirmed the high crystallinity of all membranes. Finally, I will discuss the gas separation property of these membranes. Molecular sieving is demonstrated with ideal gas selectivities up to 10.8, 6.8, 19.6, 12.3, and 14.4 for He/CH₄, H₂/CO₂, H₂/N₂, H₂/CH₄, and CO₂/N₂, respectively (Figure 1I, 1J).

Figure 1. (A) scheme of ZnC₇ and CuC₁₀ membranes formed by melting method. (B) scheme of CaC₁₂ membrane formed by interfacial crystallization. Crystal structure of ZnC₇ (C), CuC₁₀ (D) and CaC₁₂(E). Cross-sectional view SEM images of powder and the as-synthesized membranes on AAO with pore size of 100 nm: (F) ZnC₇, (G) CuC₁₀ and (H) CaC₁₂. Gas separation ideal selectivities of membranes, (I) ZnC₇ and (J) CuC₁₀, as a function of the kinetic diameters from membranes at 25 °C and 1 bar. Dash lines in (I) and (J) show the Knudsen selectivities.

References

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2. R. Corbery, Curr. Opin. Colloid In. Sci., 2008, 13, 288–302.
3. V. Luzzati, A. Tardieu, T. Gulik-Krzywicki, E. Rivas, F. Reiss-Husson, Nature 1968, 220, 1351-1352.
4. J. Peultier, E. Rocca, J. Steinmetz, Corros. Sci., 2003, 45, 1703–1716.