Syntheses and High Proton Conductivities of Two 3D Zr(IV)/Hf(IV)-MOFs from Furandicarboxylic Acid

With the gradual progress of research on proton-conductingmetal-organicframework (MOFs), it has become a challenging task to find MOF materialsthat are easy to prepare and have low toxicity, high stability, andsplendid proton conductivity. With the abovementioned objectives inmind, we selected the non-toxic organic ligand 2,5-furandicarboxylicacid and the low toxic quadrivalent metals zirconium(IV) or hafnium(IV)as starting materials and successfully obtained 2 three-dimensionalporous MOFs, [M6O4(OH)(4)(FDC)(4)(OH)(4)(H2O)(4)] [M = Zr-IV (1) and Hf-IV (2)],with ultrahigh water stability using a rapid and green synthesis approach.Their proton conductive ability is remarkable, thanks to the largenumber of Lewis acidic sites contained in their porous frameworksand the abundant H-bonding network, hydroxyl groups, as well as coordinationand crystalline water molecules. The positive correlation of theirproton conductivity with relative humidity (RH) and the temperaturewas observed. Notably, their optimized proton conductivities are 2.80x 10(-3) S & BULL;cm(-1) of 1 and 3.38 x 10(-3) S & BULL;cm(-1) of 2 under 100 & DEG;C/98% RH, which are at the forefrontof Zr(IV)/Hf(IV) MOFs with prominent proton conductivity. Logically,their framework features, nitrogen/water adsorption/desorption data,and activation energy values are integrated to deduce their protonconductivity and conducting mechanism differences. Two highly stable Zr/Hf-MOFs have beensuccessfully preparedby using a green synthesis method, which is economical, environmentallyfriendly, and rapid. The two metal-organic framework (MOFs)manifested outstanding proton conductivity above 10(-3) S & BULL;cm(-1) under 100 & DEG;C and 98% RH.