Enhanced Mechanical Stability and Proton Conductivity Performance from the Dense Mn(II)-Metal-Organic Framework to...

Postsynthetic modification (PSM) of the metal-organic framework (MOF) has been demonstrated to be an effective strategy to enhance performance. In this particular work, the anion framework Mn-MOF {[Mn₃O(H₂O)₃(HTC)]^2-} (HTC^6- = (5'-(3,5-dicarboxyphenyl)-[1,1':3',1″-terphenyl]-3,3″,5,5″-tetracarboxylate] was obtained, and NH₂(CH₃)₂⁺ ions were filled within the pores to balance the charge. In order to release the internal pores of Mn-MOF, the trivalent Fe(III) was introduced instead of Mn(II) nodes, resulting in the porous MnFe-MOF, and the NH₂(CH₃)₂⁺ ions were simultaneously deported from the pores. The content of Fe(III) in MnFe-MOF was highly dependent on the concentration of Fe(III) solution, and the maximum could be up to MnFe-MOF with a BET surface area of 1209.457 m² g^-1. Compared to the amorphization of dense Mn-MOF at 0.8 GPa in a diamond anvil cell, the mechanical stability of porous MnFe-MOF has been dramatically enhanced, and the framework integrity could be maintained up to 16.5 GPa. The proton conductivity for the MnFe-MOF series was also investigated, where MnFe-MOF showed the best performance of 1.47 × 10^-2 S cm^-1 under 70 °C and 98% RH due to the onset of reversed charge from the anionic framework to cationic framework and the formation of the most compact hydrogen bonding net. This work has not only provided an example for the PSM strategy but also illustrated that the versatile functionalities of MOF materials were mainly ascribed to the tunable porosity.