Identifying and Utilizing Upper Bounds for Diffusion-based Separations using Metal Organic Frameworks

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Proposal Number: 1604375, PI: Sholl, DavidTitle: Identifying and Utilizing Upper Bounds for Diffusion-based Separations using Metal Organic FrameworksBulk chemical separations are vital to the world-wide chemicals and fuels industries. Because these separations are largely performed with thermally-driven methods (e.g. distillation), they consume vast amounts of energy. Membrane-based separations offer a powerful alternative to conventional separations for a range of large-volume chemical separations. Commercial gas separations applications of membranes are dominated by polymeric membranes, but these membranes are limited by a fundamental trade-off between selectivity and throughput. Considerable progress has been made in recent years in making membranes from porous materials including metal-organic frameworks (MOFs). The crystalline nature of MOFs means that the fundamental tradeoff that exists for polymeric membranes can in principle be overcome. It seems likely, however, that other physical phenomena will place bounds on the ultimate performance that is possible with MOF membranes. This project will use a combination of atomically-detailed modeling and experiments to establish upper bounds on the performance of MOFs as membranes for gas separations. This work will be critical in guiding the development of these membranes for key industrial applications.The project will focus on small pore MOFs such as zeolitic imidazolate frameworks (ZIFs) and hybrid ZIFs in which separations can be achieved by both adsorption and diffusion selectivity. The work will build upon recent work by the PIs that demonstrates the ability to predict molecular diffusivities in materials of this kind using molecular simulations. These calculations rely on free energy sampling techniques, since the diffusion of species that is of practical interest occurs far too slowly for direct molecular dynamics simulations to be used. It is also vital that the models accounting for all degrees of freedom in the adsorbent framework be used to accurately describe slowly diffusing species. Detailed molecular simulations will be performed for a series of small pore MOFs that have been selected to explore hypotheses about the existence of upper bound relationships for molecular diffusion in MOFs. Simulations will be tested and validated using direct experimental measurements of diffusion in selected materials.