Mechanistic insights into lithium-ion adsorption and isotope separation on ligand-tuned aluminum-based metal–organic frameworks

Lithium isotope separation is a crucial technology with applications in nuclear energy and advanced material science. This study explores the lithium adsorption and isotope separation capabilities of aluminum-based metal–organic frameworks (Al-MOFs), notably MIL-53, DUT-4, DUT-5, MIL-120, and MIL-122, synthesized by altering organic ligands. Among them, MIL-120 demonstrated the highest lithium adsorption capacity, with a maximum uptake of 141.4 mg·g⁻¹ at 25 °C, driven by physisorption mechanisms. At the same time, DUT-5 excelled in lithium isotope separation, achieving a single-stage separation factor (α) of 1.026 and a δ (⁷Li) value of −18.03. Adsorption thermodynamics indicated that the process for MIL-120 and MIL-122 was endothermic with ΔH^θ = 58.96 kJ·mol⁻¹, while MIL-53, DUT-4, and DUT-5 followed a chemisorption pathway (ΔH^θ = 184.68 kJ·mol⁻¹). Structural and thermogravimetric analyses revealed distinct dehydration and binding behaviors, with MIL-120 facilitating surface adsorption of $\mathrm{L}\mathrm{i}({\mathrm{H}}_2{\mathrm{O})}_4^{+}$ complexes and DUT-5 promoting chemisorption through Li–O interactions. Density functional theory (DFT) calculations confirmed that electrostatic interactions with oxygen sites facilitate lithium adsorption of MIL-120, whereas DUT-5 forms stronger Al-O-Li bonds via partial dehydration. The materials exhibited excellent stability, retaining over 90 % of adsorption capacity after five regeneration cycles. These findings highlight the potential of Al-MOFs in the field of lithium isotope separation and recovery technologies.