CAREER: Fundamentals of the Relationship between Pore Structure and Transport of Light Gases in Materials with a Hierarchy of Pore Sizes

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This NSF award by the Chemical and Biological Separations program supports work by Professor Sergey Vasenkov at University of Florida to obtain fundamental knowledge of the relationship between structural properties of materials with a hierarchy of pore sizes and transport properties of gases in these materials on different length scales of molecular displacements. Porous membranes fabricated for separations of gas mixtures are usually structurally heterogeneous. In particular, mixed matrix membranes contain interconnected networks of pores of different sizes, i.e. networks of micropores with pore sizes approaching the sizes of gas molecules and networks of much larger mesopores. Such membranes have great potential for effective separations of small gas molecules such as CO2, CH4, and N2. Over the last decade significant progress has been made in understanding gas transport in pore networks with uniform or similar pore sizes. At the same time, diffusion in materials containing a hierarchy of pore networks with very different pore sizes is still poorly understood. The research goal of the project is to quantify different types of gas transport on small (i.e. micrometer and submicrometer) length scales in materials with a hierarchy of pore networks and to find a relationship between such transport and macroscopic long range diffusion. The proposed studies focus on measurements of sorbate diffusion along and through the interfaces between microporous particles and their surroundings. In addition, the effect of correlated motions of sorbate molecules on small length scales on the long range diffusion will be investigated. The hypothesis is that these types of transport can make a significant and quantifiable contribution to long range diffusion of gases in hierarchical porous materials. Measurements of gas transport will be carried out using a unique experimental approach: pulsed field gradient (PFG) NMR at high field (17.6 T) and high gradients (up to 36 T/m). Experimental studies will be complemented by dynamic Monte Carlo simulations.In the framework of the education plan of the project a new departmental program for minority undergraduate students will be introduced. The program includes organizing mentoring seminars, providing research experiences, and pairing freshmen minority students with the students who obtained research experience in the groups of the PI and other faculty members of the department. High field and high gradient PFG NMR hardware and measurement methodology will be developed as a new multi-user resource at the National Magnet Lab. The PI's plans to reach this goal include: (i) training his students to advise other users on the procedures of PFG NMR measurements, and (ii) development of a manual for such measurements. The outreach activities include expansion of the local symposium (JSEHS) for high school teachers and students and hosting 10th and 11th grade students at the PI?s group during summer months.The proposed research will quantify different types of gas transport on small length scales and establish a quantitative relationship between these types of transport and macroscopic long range diffusion in porous membranes and related materials. Molecular fluxes through macroscopic porous systems are determined by long-range diffusivities. Hence, new knowledge generated in the proposed research will enable designing concrete strategies for optimizing gas fluxes in porous membranes while preserving separation selectivities.