Modeling separation of lanthanides via heterogeneous ligand binding.

Individual lanthanide elements have physical/electronic/magnetic properties that make each useful for specific applications. Several of the lanthanides cations (Ln3+) naturally occur together in the same ores. They are notoriously difficult to separate from each other due to their chemical similarity. Predicting the Ln3+ differential binding energies (ΔΔE) or free energies (ΔΔG) at different binding sites, which are key figures of merit for separation applications, will help design of materials with lanthanide selectivity. We apply ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) to calculate ΔΔG for Ln3+ coordinated to ligands in water and embedded in metal-organic frameworks (MOFs), and ΔΔE for Ln3+ bonded to functionalized silica surfaces, thus circumventing the need for the computational costly absolute binding (free) energies ΔG and ΔE. Perturbative AIMD simulations of water-inundated simulation cells are applied to examine the selectivity of ligands towards adjacent Ln3+ in the periodic table. Static DFT calculations with a full Ln3+ first coordination shell, while less rigorous, show that all ligands examined with net negative charges are more selective towards the heavier lanthanides than a charge-neutral coordination shell made up of water molecules. Amine groups are predicted to be poor ligands for lanthanide-binding. We also address cooperative ion binding, i.e., using different ligands in concert to enhance lanthanide selectivity.

Deprotonation of formic, acetic acids and bicarbonate ion in slit silica nanopores at infinite dilution and in the presence of electrolytes.

Dielectric effects and the coupled electrostatics between the nanoconfined and the internal/external aqueous media contribute to the observed deviations of chemistry within the nanoconfined environment when compared with unconfined systems. A systematic understanding has remained elusive, especially with respect to background salt concentration and boundary condition effects like the nanopore surface chemistry and the reference state used to calculate free energies. We utilize molecular dynamics simulations along with thermodynamic integration to determine the free energy difference associated with acid-base chemistry in 2 nm and 4 nm slit pores open to a bulk-like reservoir. pKa increases are predicted when confining acetic acid, formic acid, and bicarbonate in the slits at infinite dilution conditions. We find that confinement weakens the acids, and the modulation of outer pore surface dipole magnitudes can tune the pKa shift values, suggesting that purely "intrinsic" electrostatic effect on confinement may not exist. At sufficiently high salt concentrations, the dielectric/electrostatic effects on pKa values diminish due to charge screening effects. These discoveries enable future modifications of nanopore chemistries to achieve desirable properties for industrial applications.

Bridging molecular-scale interfacial science with continuum-scale models.

Solid-water interfaces are crucial for clean water, conventional and renewable energy, and effective nuclear waste management. However, reflecting the complexity of reactive interfaces in continuum-scale models is a challenge, leading to oversimplified representations that often fail to predict real-world behavior. This is because these models use fixed parameters derived by averaging across a wide physicochemical range observed at the molecular scale. Recent studies have revealed the stochastic nature of molecular-level surface sites that define a variety of reaction mechanisms, rates, and products even across a single surface. To bridge the molecular knowledge and predictive continuum-scale models, we propose to represent surface properties with probability distributions rather than with discrete constant values derived by averaging across a heterogeneous surface. This conceptual shift in continuum-scale modeling requires exponentially rising computational power. By incorporating our molecular-scale understanding of solid-water interfaces into continuum-scale models we can pave the way for next generation critical technologies and novel environmental solutions.

Tuning the pore chemistry of Zr-MOFs for efficient metal ion capture from complex streams.

Metal-organic frameworks (MOFs) have shown promise for adsorptive separations of metal ions. Herein, MOFs based on highly stable Zr(IV) building units were systematically functionalized with targeted metal binding groups. Through competitive adsorption studies, it was shown that the selectivity for different metal ions was directly tunable through functional group chemistry.