Extended MOF-74-Type Variant with an Azine Linkage: Efficient Direct Air Capture and One-Pot Synthesis

Direct air capture (DAC) shows considerable promise for the effective removal of CO₂; however, materials applicable to DAC are lacking. Among metal-organic framework (MOF) adsorbents, diamine-Mg₂(dobpdc) (dobpdc^4- = 4,4-dioxidobiphenyl-3,3'-dicarboxylate) effectively removes low-pressure CO₂, but the synthesis of the organic ligand requires high temperature, high pressure, and a toxic solvent. Besides, it is necessary to isolate the ligand for utilization in the synthesis of the framework. In this study, we synthesized a new variant of extended MOF-74-type frameworks, M₂(hob) (M = Mg²⁺, Co²⁺, Ni²⁺, and Zn²⁺; hob^4- = 5,5'-(hydrazine-1,2-diylidenebis(methanylylidene))bis(2-oxidobenzoate)), constructed from an azine-bonded organic ligand obtained through a facile condensation reaction at room temperature. Functionalization of Mg₂(hob) with N-methylethylenediamine, N-ethylethylenediamine, and N,N'-dimethylethylenediamine (mmen) enables strong interactions with low-pressure CO₂, resulting in top-tier adsorption capacities of 2.60, 2.49, and 2.91 mmol g^-1 at 400 ppm of CO₂, respectively. Under humid conditions, the CO₂ capacity was higher than under dry conditions due to the presence of water molecules that aid in the formation of bicarbonate species. A composite material combining mmen-Mg₂(hob) and polyvinylidene fluoride, a hydrophobic polymer, retained its excellent adsorption performance even after 7 days of exposure to 40% relative humidity. In addition, the one-pot synthesis of Mg₂(hob) from a mixture of the corresponding monomers is achieved without separate ligand synthesis steps; thus, this framework is suitable for facile large-scale production. This work underscores that the newly synthesized Mg₂(hob) and its composites demonstrate significant potential for DAC applications.