Density functional theory computational study of alkali cation-exchanged sodalite-like zeolitelike metal-organic framework for CO2, N2, and CH4 adsorption

Porous adsorbents are promising for carbon capture and other industrially important gas separations, for example, CO2/N2/CH4 separations. Zeolitelike metal-organic frameworks (ZMOFs), a new subclass of MOFs, have charged frameworks, similar to conventional zeolites, which endow them with promising potential such as a high adsorption capacity and different selective molecular admission schemes from those observed in zeolites. This paper presents a density functional theory computational study of alkali cation-exchanged sodalite-like ZMOF (sod-ZMOF) for CO2, N2, and CH4 adsorption. We found that large Cs+ cations favor sites close to the pore aperture so that three Cs+ cations form a positively charged gate, controlling the admission of gas molecules. These gases have an expected sequence of binding energy values: ΔEads(CO2) > ΔEads(CH4) > ΔEads(N2). Interestingly, the energy barrier of gases passing through the gates shows an unusual sequence: ΔEa(CO2) > ΔEa(N2) > ΔEa(CH4). This sequence can be largely attributed to their energy levels at the centers of the gates formed by Cs cations. The electrostatic interaction between the positively charged gate and CO2 leads to a much higher energy level at the gate center. This is in contrast to the corresponding zeolite structures, where the apertures are enclosed by negatively charged oxygen atoms. In light of similar molecular structures at the apertures of all reported ZMOFs, our study suggests a new design route in which, by appropriate selection of extraframework cations, a unique positively charged gate can be designed that can lead to different gas admission behavior from conventional zeolite materials