Synthesis, water adsorption, and proton conductivity of solid-solution-type metal-organic frameworks al(oh)(bdc-oh)_x(bdc-nh₂) _1-x

Mixed-ligand metal-organic frameworks Al(bdc-OH)_x(bdc-NH ₂)_1-x (H₂bdc-NH₂=aminoterepthalic acid, H₂bdc-OH=hydroxyterephthalic acid) were synthesized and their water adsorption behavior and proton conductivity were investigated. All obtained compounds were isostructural to MIL-53 (MIL=Materials of Institut Lavoisier) according to XRD measurements under ambient humidity conditions, and were also found to be single phase across the whole mixing ratio from the XRD measurements under humidified conditions. This result clearly shows that all compounds are a solid-solution-type mixture of ligands. MIL-53-NH₂ adsorbs one water molecule per formula with humidification whereas MIL-53-OH adsorbs five water molecules. The mixing ratio of the ligands in Al(OH)(bdc-OH)_x(bdc-NH₂)_1-x affected the gate-opening pressure for water adsorption and total water uptake. Proton conductivity of these compounds largely depends on the adsorbed amount of water, which indicates that the proton conductivity of these compounds depends strongly on the hydrogen-bond network of the conducting media. Open the flood gates! MIL-53-type metal-organic frameworks, Al(bdc-OH)_x(bdc-NH ₂)_1-x (MIL: Materials of Institut Lavoisier, H ₂bdc-NH₂=aminoterepthalic acid, H₂bdc-OH= hydroxyterephthalic acid) consisting of a solid-solution-type mixture of ligands were obtained. The mixing ratio of the ligands affected the gate-opening pressure for water adsorption and the total water uptake (see figure). Proton conductivity of these compounds depends on the amount of adsorbed water.