Tuning Metal-Organic Framework Linker Chemistry for Transition Metal Ion Separations

The pressing demand for critical metals necessitates the development of advanced ion separation technologies for circular resource economies. To separate transition metal ions, which exhibit near-identical chemical properties, adsorbents and membranes must be designed with ultraselective chemistries. We leverage the customizability of metal-organic frameworks (MOFs) to systematically study the role of material chemistry in sorption and selectivity of Co²⁺, Ni²⁺, and Cu²⁺. To isolate the effect of MOF linker chemistry, a series of functionalized UiO-66 derivatives was synthesized from the same parent MOF through solvent-assisted linker exchange, which produced >70% linker conversion for nine linker functional groups. This work presents the first instance of post-synthetic incorporation of carboxylic acid groups in UiO-66, which was achieved with >90% conversion. A technique was developed for in situ MOF deposition in a quartz crystal microbalance to precisely monitor real-time sorption of Co²⁺, Ni²⁺, and Cu²⁺ in UiO-66-X [where X = H, (OH)₂, COOH, or (COOH)₂] and validated by comparing to batch sorption experiments (5 mM, pH 5). Carboxylic acid-functionalized derivatives exhibited the highest uptake and a trend of Cu²⁺ > Co²⁺ > Ni²⁺, with the highest sorption of 5.5% (g Cu²⁺/g MOF), equivalent to 37% mol Cu²⁺/mol linker, occurring in UiO-66-(COOH)₂. Binary salt and single salt batch sorption experiments demonstrated preferential copper binding in all studied MOFs and selectivity enhancement in binary salt conditions. UiO-66-(COOH)₂ exhibited the highest selectivity of 14 for equimolar Cu²⁺/Ni²⁺ and 13 for Cu²⁺/Co²⁺. Density functional theory calculations of ion binding energy at UiO-66-X pore windows indicate higher copper affinity for all functional groups and a trend in binding energy of UiO-66-(COOH)₂ > UiO-66-COOH > UiO-66-(OH)₂ > UiO-66-H for each transition metal ion, in good agreement with experimental results. This work highlights the effectiveness of post-synthetic modification for tuning nanostructured materials to achieve similar ion separations.