Effect of Equilibration Time and Reduced Conditions On Adsorption to 'gate-Opening Metal-Organic Frameworks'

Metal-Organic Frameworks (MOFs) which exhibit a ‘gate-opening’ (GO) phenomenon have emerged as novel adsorbents.  GO is when a flexible structure expands to accommodate gas once a thermodynamic “switch” is activated and releases the gas only when the switch is de-activated.  GO is characterized by an unusual S-shaped isotherm that shows virtually no adsorption until the switch is activated, followed by a large and steep rise in the adsorption isotherm, and ultimately, a large adsorption-desorption hysteresis. Although reproducibly demonstrated with complementary in-situ characterization to confirm structural expansion, the GO phenomenon remains poorly understood and relatively unexplored in terms of classic and emerging gas separations.  Stimulated by prior work in adsorption universality, this talk presents results for T-, P- mapping for multiple gases on two GO-MOFs. We see a pronounced effect of equilibration time on adsorption capacity, Arrhenius/anti-Arrhenius behavior, and predicted diffusion trends. Given sufficient equilibration time, GO-MOFs show convergence of capacities under similar reduced conditions. Interestingly, the capacity of a GO-MOF for a particular adsorbate demonstrates discrete values of adsorption capacity as temperature is reduced, suggesting either multiple degrees of expansion and/or packing. We explore the role of gas-surface interaction, chemical potential, and heat of adsorption on the observed trends.