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Using Membranes to Separate Molecules by Size

Speaker: John Falconer, University of Colorado, Boulder
Series: CBE Departmental Seminars
Location: Elgin Room (E-Quad A224)
Date/Time: Wednesday, September 25, 2013, 4:00 p.m. - 5:00 p.m.

Membranes can separate mixtures with much less energy than other methods, and the challenge is to prepare membranes with high selectivities, high fluxes, and long lifetimes. For gas separations, an ideal membrane should be thin and highly porous so that fluxes are high. In addition, all its pores should have the same size and shape, and the pores should be the size of molecules so that high selectivities can be obtained.  Zeolites, which are crystalline oxides that have molecular-size pores as part of their crystal structure, have the potential to form ideal membranes if micron-sized zeolite crystals can be intergrown into continuous layers without significant flow through intercrystalline regions. SAPO-34 zeolite membranes will be described that separate molecules that only differ in size by 15%; the separation is due to difference in diffusivities through the sub-nanometer size pores. These membranes have high fluxes (> 400 kg/m2 h), they can separate mixtures at high pressures (70 bar), and the zeolites are stable to high temperatures. Thus, they have the potential for large-scale applications. SAPO-34 membranes can also separate gas mixtures by molecular sieving when one molecule in a mixture is larger than the SAPO-34 pores, and we have measured separation selectivities greater than 10,000 for various mixtures. These membranes can be made selective for hydrogen by adding a 10-nm thick layer of porous alumina by molecular layer deposition, and the results of this modification will be presented.