Design of a robust and strong-acid MOF platform for selective ammonium recovery and proton conductivity

Metal-organic frameworks (MOFs) are potential candidates for the platform of the solid acid; however, no MOF has been reported that has both aqueous ammonium stability and a strong acid site. This manuscript reports a highly stable MOF with a cation exchange site synthesized by the reaction between zirconium and mellitic acid under a high concentration of ammonium cations (NH₄⁺). Single-crystal XRD analysis of the MOF revealed the presence of four free carboxyl groups of the mellitic acid ligand, and the high first association constant (pK_a1) of one of the carboxyl groups acts as a monovalent ion-exchanging site. NH₄⁺ in the MOF can be reversibly exchanged with proton (H⁺), sodium (Na⁺), and potassium (K⁺) cations in an aqueous solution. Moreover, the uniform nanospace of the MOF provides the acid site for selective NH₄⁺ recovery from the aqueous mixture of NH₄⁺ and Na⁺, which could solve the global nitrogen cycle problem. The solid acid nature of the MOF also results in the proton conductivity reaching 1.34 × 10^-3 S cm^-1 at 55 °C by ion exchange from NH₄⁺ to H⁺.