Understanding gate adsorption behaviour of CO2 on elastic layer-structured metal-organic framework-11

We demonstrate that CO2 gate adsorption behaviour of elastic layer-structured metal-organic framework-11 (ELM-11: [Cu(BF4)2(4,4'-bipyridine)2]), which is a family of soft porous crystals (SPCs), can be described by a thermodynamic model by free energy analysis with the aid of an adsorption experiment and a molecular simulation. The structures of ELM-11 (closed structure) at 273 K after its evacuation and CO2-encapsulated ELM-11 (open structure) at 195-298 K were determined by the Rietveld analysis using in situ synchrotron X-ray powder diffraction data. We then performed grand canonical Monte Carlo (GCMC) simulations for CO2 adsorption on the open host framework structures of ELM-11 from the Rietveld analysis. The temperature dependence of the Helmholtz free energy change of host ΔF(host) from the closed structure to the open structure was obtained by the free energy analysis using the GCMC data. We show that there is a linear correlation between ΔF(host) and temperature, and thus, the internal energy and entropy changes of host, ΔU(host) and ΔS(host), respectively, can be obtained. The obtained ΔU(host) value is in good agreement with that obtained from the quantum chemical calculations using the closed and open host framework structures, which demonstrates that the thermodynamic model for gate adsorption is highly appropriate. Moreover, our result suggests that the gate adsorption pressure depends on not only the guest-host interaction and the internal energy change of host, but also the entropy change of host, which should be one of the key factors for the tailored synthesis of SPCs.