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 2 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 2 for selective CO 2 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 (H 2 Me 2 bipy) was used to prepare a Zr-based MOF [MOF-553, Zr 6 O 4 (OH) 4 (Me 2 Bipy) 6 ]. In addition, the water stability and CO 2 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 2 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 2 capture. MOF-553 with its 6,6′-dimethyl-2,2′-bipyridine-5,5′-dicarboxylate linker is more robust and has a higher CO 2 adsorption capacity than the 2,2′-bipyridine-5,5′-dicarboxylate containing UiO-67(bipy) MOF.