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.