Thermodynamics of alkali metal ion uptake from aqueous solution in MOF-808

Abstract

The growing global demand for critical metals has intensified the search for sustainable and efficient extraction methods. Passive adsorption from seawater using advanced sorbent materials has emerged as a promising alternative, offering a renewable and environmentally responsible resource. Metal–organic frameworks (MOFs), with their high surface area and tunable pore structures, offer great potential for selective ion uptake; however, a molecular-level understanding of ion adsorption from dilute aqueous solutions remains incomplete. In this study, we employ free-energy calculations and enhanced sampling simulations to investigate alkali metal ion uptake in MOF-808, a prototypical hydrothermally stable MOF. Our results reveal that large pores provide a similarly stable environment for all studied ions, indicating a lack of intrinsic selectivity, whereas small pores exhibit distinct thermodynamic and kinetic preferences that govern ion uptake. Dehydrated alkali metal ions are stable within small pores, and free-energy profiles reveal that their transfer from large to small pores occurs with lower energy barriers than that of water molecules. Among these ions, Li$^+$ faces the highest barrier due to its strong hydration shell, whereas K$^+$ exhibits the greatest thermodynamic preference for uptake in its dehydrated state. However, within hydrated small pores, Li$^+$ is the most stable, underscoring the interplay between hydration structure and confinement effects. These findings provide fundamental insights into ion uptake in MOFs and offer guidance for designing next-generation MOFs with enhanced selectivity for metal ion extraction from dilute solutions. Future efforts should explore pore functionalization and tailored confinement strategies to optimize MOFs for efficient and selective metal recovery.