Enhanced water stability and high CO2 storage capacity of Lewis basic sites containing zirconium metal–organic framework

Metal–organic frameworks (MOFs) are an emerging class of materials employed for custom-designed purposes by judicious selection of the linker and the metal ions. Among the MOFs composed of carboxylate linkers, Zr-based MOFs have attracted great attention due to their high thermal and chemical stability, which are important for practical applications, including capturing CO2 from a point source. UiO-67(bipy) containing 2,2’-bipyridine-5,5’-dicarboxylate is particularly useful among the Zr-MOF family due to the Lewis basic sites of the linker; however, hydrolytic stability of UiO-67(bipy) does not seem to be as high as UiO-66 and UiO-67. To improve hydrolytic stability without sacrificing the adsorption enthalpy of CO2 for selective CO2 capture, in this study, we added hydrophobic methyl groups to the backbone of the bipyridine linker. Synthesized 6,6’-dimethyl-2,2’-bipyridine-5,5’-dicarboxylic acid (H2Me2bipy) was used to prepare a Zr-based MOF [MOF-553, Zr6O4(OH)4(Me2Bipy)6]. Additionally, the water stability and CO2 adsorption capacity of MOF-553 were compared to UiO-67(bipy). We revealed that MOF-553 is more robust and has a higher CO2 adsorption capacity than UiO-67(bipy), indicating that the methylation of the linker improves the water-stability of the framework, which is advantageous for the point source CO2 capture.