Engineering Li/Na selectivity in 12-Crown-4–functionalized polymer membranes

Abstract

Lithium is widely used in contemporary energy applications, but its isolation from natural reserves is plagued by time-consuming and costly processes. While polymer membranes could, in principle, circumvent these challenges by efficiently extracting lithium from aqueous solutions, they usually exhibit poor ion-specific selectivity. Toward this end, we have incorporated host–guest interactions into a tunable polynorbornene network by copolymerizing 1) 12-crown-4 ligands to impart ion selectivity, 2) poly(ethylene oxide) side chains to control water content, and 3) a crosslinker to form robust solids at room temperature. Single salt transport measurements indicate these materials exhibit unprecedented reverse permeability selectivity (∼2.3) for LiCl over NaCl—the highest documented to date for a dense, water-swollen polymer. As demonstrated by molecular dynamics simulations, this behavior originates from the ability of 12-crown-4 to bind Na+ ions more strongly than Li+ in an aqueous environment, which reduces Na+ mobility (relative to Li+) and offsets the increase in Na+ solubility due to binding with crown ethers. Under mixed salt conditions, 12-crown-4 functionalized membranes showed identical solubility selectivity relative to single salt conditions; however, the permeability and diffusivity selectivity of LiCl over NaCl decreased, presumably due to flux coupling. These results reveal insights for designing advanced membranes with solute-specific selectivity by utilizing host–guest interactions.

Origins of Lithium/Sodium Reverse Permeability Selectivity in 12-Crown-4-Functionalized Polymer Membranes

“…Within the context of the solution-diffusion model, permeability selectivity is expressed as the product of solubility selectivity ( K LiCl / K NaCl ) and diffusivity selectivity ( D LiCl / D NaCl ), as shown in eq . Based on previous studies, permeability selectivity in 12C4-functionalized membranes is largely dictated by diffusivity selectivity . Moreover, computational approaches for probing ion solubility are still less well developed than methods for characterizing ion transport.…”

“…The experimental system being modeled is a statistical copolymer with a poly­(norbornene) (PNBE) backbone containing a pendant 12C4 functionality to promote ion selectivity, cross-linker, and oligimeric poly­(ethylene glycol) (PEG) side chains that allow for tunable membrane water content . Here, we chose to model only the selective portion of the polymer with degree of polymerization N = 20.…”

“…Here, we chose to model only the selective portion of the polymer with degree of polymerization N = 20. This assumes that the PEG chains do not impart selective interactions with ions, which is not rigorously true, as poly­(ethylene glycol diacrylate) (PEGDA) membranes have a slight selectivity for LiCl over NaCl. , However, the loading of PEG in these membranes is relatively small (0–10 wt %), and thus the omission of PEG is likely reasonable. Furthermore, selectivity in PEGDA membranes results from solubility effects, not diffusivity effects, which are the focus of this study. , …”

“…In a recent study, a polymer membrane containing 12-crown-4 ether (12C4) was synthesized, and it had a LiCl/NaCl permeability selectivity (denoted by the ratio of salt permeability coefficients P LiCl / P NaCl ) of 2.3, the largest permeability selectivity in a fully aqueous membrane system reported to date. In the original work, the permeability selectivity of LiCl over NaCl was demonstrated through simulations to result from strong, favorable interactions between Na + and 12C4.…”

Designing polymeric membranes with coordination chemistry for high-precision ion separations

“…The prospect of bypassing the permeability-selectivity trade-off has led to extensive research in molecular sieving materials, such as carbon nanotubes and two-dimensional materials, as ion-selective membranes ( 10 , 14 ). Comparatively, much less attention has been given to tuning the coordinative interactions between a species of interest and membrane matrix—one key feature that purportedly imparts biological membranes with their remarkable selectivity ( 15 , 16 ).…”