(21c) Understanding Mass Transfer of Gases in Porous Materials Using Frequency Response

Porous materials have been proposed to solve a multitude of complex problems, such as CO₂ separation and sequestration, but in many cases when new materials are reported only equilibrium capacity measurements are presented. While helpful, equilibrium isotherms do not provide insight into the rate by which adsorption takes place. Several techniques have been developed to examine the mass transfer of adsorbates into porous materials including zero-length column approaches, gravimetric uptake methods, and NMR techniques. Of the various methods, concentration swing frequency response (CSFR) is particularly helpful when examining the mass transfer rates of gases into novel porous solids because only milligram amounts of adsorbent material are required, and transport diffusion rates can be measured. The technique allows for the measurement of light gases and condensable vapors and can distinguish the mechanism of adsorption in the material. This talk will present the application of frequency response to understand the mass transport of gases in porous materials. The CSFR method will be reviewed and example CSFR data will be shown detailing the rate of adsorption on traditional adsorbent materials. Next, the utilization of CSFR to characterize water vapor diffusion into metal-organic frameworks and the applicability of the Darken equation to these systems will be discussed. Lastly, the ability of CSFR to distinguish minor changes in MOF functional groups will be discussed and the impact of this technique on the development of new materials will be considered.