Rational designs for highly proton-conductive metal-organic frameworks

(Graph Presented) A novel metal-organic framework (MOF), (NH ₄)₂(adp)[Zn₂(ox)₃]·3H ₂O (1) was synthesized and its structure was determined. We propose three types of rational design to introduce proton carriers into MOFs. The simplest method is to introduce them directly as counterions such as NH ₄⁺, H₃O⁺, and HSO₄ ^- into the pores of frameworks (type I). The second is to put acid groups on frameworks, the protons being provided from them (type II). The third is to incorporate acidic molecules into voids (type III). 1 demonstrated a combination of two of the concepts by introducing NH₄⁺ ions using the anionic framework (type I) and putting carboxyl end groups of adipic acid in a honeycomb-shaped void (type III). 1 showed a superprotonic conductivity of 10^-2 S cm^-1 at ambient temperature, comparable to organic polymers such as Nafion, which is in practical use in fuel cells. This is the first example of an MOF to exhibit a superprotonic conductivity of 10^-2 S cm^-1 at ambient temperature.