Ammonia as Proton Conducting Medium Confined in Porous Materials

Abstract

<b></b><p>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.H<sub>2</sub>O, efficient conducting medium, confined in hydrophobic channels forms unique clusters allowing rapid diffusion, whereas confined NH<sub>3</sub>, having a similar degenerate system (Fig. 1a), has not been reported. Herein, we show NH<sub>3</sub>-mediated proton conduction in microporous metal–organic frameworks (MOFs), MIL-53(Al) functionalized with (-COOH)<sub>2</sub>, -NH<sub>2</sub>, -OH and -H. Anhydrous NH<sub>3 </sub>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 NH<sub>3</sub>.</p><br>

Proton transfer in hydrogen-bonded degenerate systems of water and ammonia in metal–organic frameworks

“…21). 76 As mentioned above, pure NH 3 molecules are converted to NH 4 + by protonation from the dangling –(COOH) 2 in the ligand. Furthermore, one of the NH 3 molecules has an interaction with the μ 2 -OH bridge.…”

“…Previous studies have demonstrated NH 3 (NH 4 + )-mediated proton conductors through the simultaneous introduction of the NH 4 + counterion during MOF synthesis or aqueous-NH 3 vapor loading exhibiting an improved proton conductivity. Anhydrous NH 3 -mediated proton conductivity was reported recently using MIL-53(Al) derivative MOFs through pure NH 3 gas adsorption 76. MIL-53 (Al) derivatives that are well known for their high stability are synthesized from Al III ions and H 2 bdc ligands with various functional groups, (H 2 bdc–(COOH) 2 , 1 ), (H 2 bdc–(NH 2 ), 2 ), (H 2 bdc–(OH), 3 ), and (H 2 bdc, 4 ), indicating the difference in proton-donating ability (p K a ).…”