Double-Step Gate Phenomenon in CO₂ Sorption of an Elastic Layer-Structured MOF

A double-step CO₂ sorption by [Cu­(4,4′-bpy)₂(BF₄)₂] (ELM-11) was observed during isothermal measurements at 195, 253, 273, and 298 K and was accompanied by interlayer expansion in the layered structure of ELM-11. The first step occurred in the range of the relative pressure (P/P₀) from 10^–3 to 10^–2. The second step was observed at P/P₀ ≈ 0.3 at the four temperatures. Structural changes in ELM-11 during the CO₂ sorption process were examined by X-ray diffraction (XRD) measurements. The structural change for the first step was well understood from a detailed structural analysis, as reported previously. The XRD results showed further expansion of the layers during the second step as compared to the already expanded structure in the first step, and both steps were found to be caused by the gate phenomenon. The energy for the expansion of the layer structure was estimated from experimental and simulated data.