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.