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

Abstract

Direct lithium extraction via membrane separations has been fundamentally limited by lack of monovalent ion selectivity exhibited by conventional polymeric membranes, particularly between sodium and lithium ions. Recently, a 12-Crown-4-functionalized polynorbornene membrane was shown to have the largest lithium/sodium permeability selectivity observed in a fully aqueous system to date. Using atomistic molecular dynamics simulations, we reveal that this selectivity is due to strong interactions between sodium ions and 12-Crown-4 moieties, which reduce sodium ion diffusivity while leaving lithium ion mobility relatively unaffected. Moreover, the ion diffusivities in the membrane, when scaled by their respective solution diffusivities and free ion fractions, can be collapsed to an almost universal relationship depending on solvent volume fraction.

Impact of Cation–Ligand Interactions on the Permselectivity of Ligand-Functionalized Polymer Membranes in Single and Mixed Salt Systems

“…In other words, consecutive ligands on a polymer chain were separated by two non-interacting beads. This choice of spacer length yielded a ligand to polymer volume ratio of 0.36, which is very similar to the value (0.4–0.45) used in the MD simulations of 12-crown-4 functionalized poly­(norborene) membranes by Warnock et al , Both single and mixed salt systems were probed in this study. Salt molecules were modeled explicitly via ion pairs.…”

“…This can be a critical bottleneck hindering the mainstream use of polymer membranes for lithium purification. One way to circumvent this problem is by functionalizing the polymer membranes with ligands to incorporate host–guest interactions within the membrane. − The ligands exhibit different binding affinity to the various ions, thereby creating an inherent asymmetry in the ionic transport process, that is, ions that bind strongly to the ligands exhibit different transport behavior as compared to ions that do not interact with the ligands. The potassium channels in our bodies are able to achieve highly selective transport of K + over Na + by leveraging these host–guest interactions .…”

Recent Advances in the Lithium Recovery from Water Resources: From Passive to Electrochemical Methods

“…and literature works that is still performed to this day to better understand its potentialities. [ 178,194–200 ] In the following, several examples of different materials and morphologies of lithium adsorbents employing mostly 12‐crown‐4 ether for lithium recovery are presented.…”

Crown-Ether Functionalized Graphene Oxide Membrane for Lithium Recovery from Water

“…Works of dual crown ether functionalized to successfully recover two different cations at once have also be reported [30]. The family of 12-crown 4-ethers is known to be extremely selective toward lithium [31][32][33][34][35][36][37][38] due to the perfect fit of its cavity with the ionic radius of the lithium ion and is therefore used in this approach. Recently, their possible application in lithium recovery has been investigated by grafting them on polymers or graphene oxide to realize either ion-imprinted polymers or membranes [30,35,36,[39][40][41][42][43].…”

“…The family of 12-crown 4-ethers is known to be extremely selective toward lithium [31][32][33][34][35][36][37][38] due to the perfect fit of its cavity with the ionic radius of the lithium ion and is therefore used in this approach. Recently, their possible application in lithium recovery has been investigated by grafting them on polymers or graphene oxide to realize either ion-imprinted polymers or membranes [30,35,36,[39][40][41][42][43]. Several polymers have been tested for these kinds of applications, the most common being poly(vinylidene fluoride) (PVDF) [39,40,42] and polysulfone [44] or natural polymers [37].…”