Abstract
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 formate dynamics increase with temperature, new kinetic adsorption regimes for an adsorbate can be accessed, marked by kinetic inflection temperatures (KITs). Identification of these temperatures allows kinetic or absolute gating separations to be devised without exhaustive experimentation. However, though an elevated temperature regime may promote fast diffusion for an adsorbate, adsorption quantities can be minimal if thermodynamics of adsorption have been overcome. 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.
| Original language | English |
|---|---|
| Pages (from-to) | 7102-7114 |
| Number of pages | 13 |
| Journal | Chemistry of Materials |
| Volume | 37 |
| Issue number | 18 |
| DOIs | |
| State | Published - Sep 23 2025 |
Funding
The authors gratefully acknowledge the partial support from following sources. The National Institute of Standards and Technology (H.A.E., T.Y., G.A.M., T.T.M., A.R.H.W., C.M.B.). The Ras al Khaimah Centre for Advanced Materials (A.K.C.). The Lee Kuan Yew Postdoctoral Fellowship (22-5930-A0001) (Z.D.). The Robert A. Welch Foundation under grants L-E-001-19921203, and additional financial support from the Welch Foundation under award E-2227- 20250403 (P.C.). The NIST Postdoctoral Program for an NRC Postdoctoral Fellowship (G.A.M.). This research used beamline 28-ID-2 of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. This research utilized powder X-ray diffraction data collected at beamline 17-BM at the Advanced Photon Source at the Argonne National Laboratory, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under contract DEAC02-06CH11357. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. The computing resources for DFT phonon and VISION spectra simulations were made available through the VirtuES and the ICE-MAN projects, funded by Laboratory Directed Research and Development program and Compute and Data Environment for Science (CADES) at ORNL.