Enhanced water stability and high CO storage capacity of a 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 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 CO 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 CO for selective CO 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 (HMebipy) was used to prepare a Zr-based MOF [MOF-553, ZrO(OH)(MeBipy)]. In addition, the water stability and CO 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 CO 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 CO capture.