Tuning carbon dioxide capture capability with structural and compositional design in mmen‐(Mg,Zn) (dobpdc) metal‐organic framework: density functional theory investigation

This paper presents a materials design for tuning the CO2‐capture capability of mixed‐metal‐organic framework mmen‐(ZnxMg1‐x)2(dobpdc) using density functional theory. The structural stability of a mixed‐metal structure with respect to its parent single‐metal species was investigated via energy of mixing (ΔEmix). We found that the ΔEmix of all mixed‐metal structures are negative, signifying exothermic mixing. This then indicates that mixed‐metal structures are energetically preferable compared with their non‐interacting single‐metal species. Moreover, the magnitudes of ΔEmix of all mixed‐metal structures are lower than 1 kJ/mol, implying an ‘ideal mixing’. Density‐of‐state analysis also reveals that the electronic structures of both Mg and Zn in mixed‐metal structures are not significantly different from those in parent single‐metal species, suggesting weak chemical interaction between Mg and Zn in mixed‐metal structures. For CO2‐adsorbed mmen‐(ZnxMg1‐x)2(dobpdc), we found that the adsorption energy (Eads) is a linear function of mixing ratio (x), and does not depend on how Mg and Zn atoms are arranged in forming different mixed‐metal structures. This can be described by the weak chemical interaction between Mg and Zn in a mixed‐metal structure. Consequently, Eads can then be predicted conveniently from the mixing ratio, while the adsorption properties of parent single‐metal species can be tuned with the proposed mixed‐metal method. We then expect that the knowledge of materials design using a mixed‐metal approach presented in this work would benefit the community in providing the ability to tune CO2 capture capability efficiently in the materials studied. © 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.