Modeling and Visualization of CO₂ Adsorption on Elastic Layer-Structured Metal–Organic Framework-11: Toward a Better Understanding of Gate Adsorption Behavior

Elastic-layer-structured metal–organic framework-11 (ELM-11) is a soft porous crystal (SPC) with a 2D square-grid framework [Cu­(BF₄)₂(bpy)₂] (bpy = 4,4′-bipyridine) that has attracted attention as a material for CO₂ capture and storage because of its gate adsorption properties. Herein, we demonstrate that the structure of CO₂-encapsulated ELM-11 at 100 kPa and 273 K can be precisely modeled and visualized by our new structure refinement method, which combines Rietveld analysis of in situ synchrotron X-ray powder diffraction data with molecular simulations. We believe that this is the first study in which the structure of a guest–SPC system that exhibits an extensive, complex structural transformation by the gate adsorption was successfully refined by such an approach. The crystallographic data of the open framework structure of ELM-11 enable grand canonical Monte Carlo (GCMC) simulations of CO₂ adsorption. The free energy analysis of the gate adsorption phenomenon with the resulting GCMC adsorption isotherm of CO₂ provides the precise Helmholtz free energy change of the host framework during the structural transition, which is difficult to access experimentally. Finally, we demonstrate that the temperature dependence of the gate adsorption pressure can be predicted using the Helmholtz free energy change of the host.