Ammonia as Proton Conducting Medium Confined in Porous Materials

Molecular confinement within a limited space induces unique behaviour not seen in bulk systems. In particular, the proton diffusion in conducting medium under confined conditions is significantly affected by the surrounding environment.H₂O, efficient conducting medium, confined in hydrophobic channels forms unique clusters allowing rapid diffusion, whereas confined NH₃, having a similar degenerate system (Fig. 1a), has not been reported. Herein, we show NH₃-mediated proton conduction in microporous metal–organic frameworks (MOFs), MIL-53(Al) functionalized with (-COOH)₂, -NH₂, -OH and -H. Anhydrous NH₃ gas is trapped in the pore by proton donation of frameworks and forms hydrogen bonding networks exhibiting a remarkably enhanced proton conductivity. The crystallographic analysis and solid-state NMR clarify the veiled proton diffusion mechanism and unique dynamic behaviour of confined NH₃.