Miao Yu
Department of Chemical and Biological Engineering
University of Colorado, Boulder

ABSTRACT Nanoporous membranes have shown great potential on separating mixtures in an energy-efficient way. Both energy-related mixture separations, such as natural gas purification and alcohol/water separation, and environment-related separations, such as CO2 capture and water treatment, have been widely investigated using membrane based separation processes. Very promising results have been obtained. For example, high quality SAPO-34 zeolite membranes (0.38 nm crystal pores) separated CO2 from CH4 efficiently up to pressure drop 7 MPa with separation selectivity higher than 50; all-silica form MFI zeolite membranes(0.55 nm crystal pores)  separated ethanol from water with a separation selectivity higher than 100. My research work concentrates on two types of nanoporous inorganic membranes: zeolite membranes and dense, vertical-aligned carbon nanotube membranes.

With uniform, molecular-sized pores and excellent stabilities (chemical, thermal and mechanical), zeolites/molecular sieves are perfect membrane materials to realize separations by molecular sizes. However, it is a big challenge to make thin, defect-free zeolite membranes because of the polycrystalline feature of zeolite membranes and thus inevitable intercrystalline pores/defects; sparsely distributed (several volume percent), nanometer-sized defects (< 5nm) could essentially “kill” a zeolite membrane. Various methods, including optimizing hydrothermal growth conditions, matching crystal growth with porous supports and selectively blocking defects etc., have been  used to minimize defects amount and defects sizes in zeolite membranes. To evaluate the efficiency of these methods, microstructure change of zeolite membranes must be known before and after. This requires understanding of the microstructure of zeolite membranes. One direction of my research work is focused on understanding 3 aspects of the microstructure: What is the percentage of flow through defects? What are defects sizes? Is the microstructure rigid or flexible and could we manipulate the microstructure? My answers to these questions will be given in the talk.

Carbon nanotubes (CNT) are emerging membrane materials. One main motivation of preparing carbon nanotube membranes is the predicted high fluxes in CNTs by molecular dynamics. The predicted high fluxes have been experimentally confirmed using composite CNT membranes, which use polymeric or inorganic sealing materials between CNTs. The previous composite CNT membrane preparation procedures are complicated and <3% of the composite CNT membranes are permeable. This means >97% of membrane area is impermeable! Towards practical application of CNT membranes, we developed a simple technique to fabricate high volume density (~ 20vol.% CNTs), aligned CNT membranes. No sealing materials are necessary because pores between CNTs have similar sizes as CNTs. This introduces extra transport pathway. Structural characterization and size exclusion measurements indicate high quality of dense CNT membranes with membrane cut-off pore size ~ 3 nm. Nitrogen permeability through dense CNT membranes is 4 to 7 orders of magnitude higher than previous composite CNT membranes due to much denser CNT packing and extra transport pathway between CNTs. Potential applications of dense CNT membranes include water nano-filtration, air purification and high flux gas separations.

BIO Dr. Miao Yu is a research associate in the Department of Chemical and Biological Engineering at the University of Colorado at Boulder. He received his Ph.D. in Chemical Engineering from the University of Colorado at Boulder in 2007. Prior to being a Ph.D. student at the University of Colorado at Boulder, Dr. Yu was a Ph.D. candidate in Chemical Engineering at the University of Minnesota in 2004.  His research focuses on synthesis, characterization and novel applications of nanoporous inorganic membranes, applications of atomic layer deposition (ALD) and molecular layer deposition (MLD) and dye-sensitized solar cells (DSSC).