Effect of Imidazole Arrangements on Proton-Conductivity in Metal–Organic Frameworks

Imidazole
molecules were frequently incorporated into porous materials
to improve their proton conductivity. To investigate how different
arrangements of imidazoles in metal–organic frameworks (MOFs)
affect the overall proton conduction, we designed and prepared a MOF-based
model system. It includes an Fe–MOF as the blank, an imidazole@Fe–MOF
(Im@Fe–MOF) with physically adsorbed imidazole, and an imidazole–Fe–MOF
(Im–Fe–MOF), which contains chemically coordinated imidazole
molecules. The parent Fe–MOF, synthesized from the exchange
of carboxylates in the preformed [Fe3(μ3–O)]­(carboxylate)6 clusters and multitopic carboxylate
ligands, serves as a control. The Im@Fe–MOF was prepared by
encapsulating free imidazole molecules into the pores of the Fe–MOF,
whereas the Im–Fe–MOF was obtained in situ, in which
imidazole ligands coordinate to the metal nodes of the framework.
Proton-conductivity analyses revealed that the proton conductivity
of Im–Fe–MOF was approximately two orders of magnitude
greater than those of Fe–MOF and Im@Fe–MOF at room temperature.
The high proton conductivity of 1.21 × 10–2 S cm–1 at 60 °C for Im–Fe–MOF
ranks among the highest performing MOFs ever reported. The results
of the density functional theory calculations suggest that coordinated
imidazole molecules in Im–Fe–MOF provide a greater concentration
of protons for proton transportation than do coordinated water molecules
in Fe–MOF alone. Besides, Im–Fe–MOF exhibits
steadier performance than Im@Fe–MOF does after being washed
with water. Our investigation using the above ideal crystalline model
system demonstrates that compared to disorderly arranged imidazole
molecules in pores, the immobilized imidazole molecules by coordination
bonds in the framework are more prone to form proton–conduction
pathways and thus perform better and steadier in water-mediated proton
conduction.