With the gradual progress of research on proton-conducting
metal–organic
framework (MOFs), it has become a challenging task to find MOF materials
that are easy to prepare and have low toxicity, high stability, and
splendid proton conductivity. With the abovementioned objectives in
mind, we selected the non-toxic organic ligand 2,5-furandicarboxylic
acid and the low toxic quadrivalent metals zirconium(IV) or hafnium(IV)
as starting materials and successfully obtained 2 three-dimensional
porous MOFs, [M6O4(OH)4(FDC)4(OH)4(H2O)4] [M = ZrIV (1) and HfIV (2)],
with ultrahigh water stability using a rapid and green synthesis approach.
Their proton conductive ability is remarkable, thanks to the large
number of Lewis acidic sites contained in their porous frameworks
and the abundant H-bonding network, hydroxyl groups, as well as coordination
and crystalline water molecules. The positive correlation of their
proton conductivity with relative humidity (RH) and the temperature
was observed. Notably, their optimized proton conductivities are 2.80
× 10–3 S·cm–1 of 1 and 3.38 × 10–3 S·cm–1 of 2 under 100 °C/98% RH, which are at the forefront
of 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 proton
conductivity and conducting mechanism differences.