Functionalized Base-Stable Metal-Organic Frameworks for Selective CO2 Adsorption and Proton Conduction

Abstract

Metal-organic frameworks (MOFs) have shown great potential for application in various fields, including CO₂ capture and proton conduction. For promoting their practical applications, both optimization of a given property and enhancement of chemical stability are crucial. In this work, three base-stable isostructural MOFs, [Ni₈ (OH)₄ (H₂ O)₂ (BDP-X)₆ ] (Ni-BDP-X; H₂ BDP=1,4-bis(4-pyrazolyl)benzene, X=CHO, CN, COOH) with different functional groups, are designed, synthesized, and used in CO₂ capture and proton conduction experiments. They possess face-centered cubic topological structures with functional nanoscale cavities. Importantly, these MOFs are fairly stable to maintain their structures in boiling water and 4 M sodium hydroxide solution at room temperature. Functionalization endows them with tunable properties. In gas adsorption studies, these MOFs exhibit selective adsorption of CO₂ over CH₄ and N₂ , and in particular the introduction of COOH groups provides the highest selectivity. In addition, the COOH-functionalized Ni-BDP exhibits a high proton conductivity of 2.22×10^-3 S cm^-1 at 80 °C and approximately 97 % relative humidity.

Keywords: Ni8 clusters; chemical stability; polypyrazolates; proton conduction; selective CO2 adsorption.