Diffusional Priority Enables Kinetic ⁶Li Selectivity in a Covalent Organic Framework

Lithium isotope (⁶Li/⁷Li) separation is hindered by nearly identical electronic structures and strong solvation. Adsorption strategies based on ligand-ion thermodynamic affinity often struggle to explain the experimentally observed preference for ⁶Li over ⁷Li. A crown ether-functionalized covalent organic framework is employed as a representative adsorption system to distinguish the contributions of equilibrium binding and transport processes. Density functional theory calculations show that amino-benzo-15-crown-5 exhibits a slight thermodynamic preference for ⁷Li, with a lower Gibbs free energy of complexation. Batch experiments reveal an unexpected preference for ⁶Li adsorption. This behavior arises from its higher zero-point energy and lower mass, which facilitate desolvation and accelerate diffusion, leading to kinetically favored coordination over thermodynamically preferred ⁷Li. Once coordinated, the small Gibbs free energy difference between isotopes provides limited driving force for ligand exchange, hindering replacement of ⁶Li by ⁷Li. These findings support the important role of nonequilibrium transport processes in adsorption-based lithium isotope separation.