Temperature-Regulated Gating Enables Gas Separations in Ultramicroporous Aluminum Formate, ALF

Physisorption is a reversible exothermic phenomenon where molecular kinetic energy is limited and interactions between guest molecules and materials are favored at low temperatures. However, in certain ultramicroporous materials, physisorption can be impacted by subtle structural changes on decreasing temperature that slows or even stops adsorbate diffusion, circumventing thermodynamic expectations. These unique ultramicroporous materials are described as temperature-regulated gating adsorbents and given their special properties, can facilitate mix-and-match gas separations by simply controlling temperature. To date, though understood to be remarkably useful, there is still ambiguity about how best to identify, characterize, and rationalize the performance of these materials. To address this issue, we provide a practical analytical framework of a model gating material, Al(HCOO)3, ALF. Our work illustrates how the gating effect in ALF originates from the changing dynamics of the formate linkers that define the apertures between porous cavities. As the dynamics of the formates increase with temperature, new kinetic adsorption regimes for an adsorbate can be accessed, marked by kinetic inflection temperatures (KITs), which correspond to adsorbate-specific kinetic barriers encountered during diffusion. Identifying these temperatures is essential, as they allow kinetic or absolute gating separations to be devised without exhaustive experimentation. However, though a temperature regime may facilitate favorable diffusion kinetics for certain adsorbates, the thermodynamic driving forces for adsorption may have already been overcome making adsorption quantities minimal. By using gas sorption studies with noble gases, H2, N2, O2, CO2, and C2H2, as well as crystallography, spectroscopy, and modeling, our work elucidates how the convoluted effects of thermodynamics and kinetics affect a system like ALF, and how they can be leveraged for separation design. We do this by rationalizing the origin of temperature-regulated gating in ALF and showing how targeted density functional theory calculations and routine temperature programmed desorption experiments can predict real-world conditions for untested gas separations. Our work serves as a guide to the future characterization of suspected temperature-regulated gating materials, and, more generally, ultramicroporous materials that may inherently possess this behavior.