Enhanced water stability and high CO₂ storage capacity of a Lewis basic sites-containing zirconium metal-organic...

Metal-organic frameworks (MOFs) are an emerging class of materials employed for custom-designed purposes by judicious selection of linkers and metal ions. Among the MOFs composed of carboxylate linkers, Zr-based MOFs have attracted great attention due to their high thermal and chemical stabilities, 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, the hydrolytic stability of UiO-67(bipy) does not seem to be as high as those of UiO-66 and UiO-67. To improve the 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. The 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]. In addition, the water stability and CO₂ adsorption capacity of MOF-553 were compared to those of 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 point-source CO₂ capture.