Lithium Selectivity of Crown Ethers : The Effect of Heteroatoms and Cavity Size

Lithium is a metal in increasingly high demand due to its use in lithium-ion batteries. One of the largest sources of lithium are aqueous resources such as brines or seawater. However, the extraction of this metal is still challenging due to the presence of other interfering metal ions. Crown ethers (CEs) are known to complex metal cations very effectively, but a fundamental understanding of their potential in lithium extraction is still lacking, also when considering derivatives of CEs with different heteroatoms. Therefore, the selective complexation of Li+ over other alkaline (Na+, K+) and alkaline earth metal (Mg2+, Ca2+) ions is investigated by density functional theory (DFT) calculations for 15-, 12-, and 9-membered CEs and their derivatives containing in part nitrogen and sulfur instead of oxygen atoms. Structure optimizations of these CEs are performed in vacuum, and complex stabilities are discussed by evaluating the cavity size, the distances between donor atoms and metal ion, and by performing Hirshfeld charge and Natural Bond Orbital analyses. The qualitative trends obtained from these methods were in a good agreement with the complex stabilities. This suggests that they can used to make simplified predictions of complex stabilities and to analyze them. The most stable Li+ complex was achieved for the 15-membered CE by exchanging a single oxygen donor atom with sulfur. In case of the 12- and 9-membered rings the Li+ complex stability is at the highest when oxygen is replaced by nitrogen. Furthermore, it was shown that the selectivity of Mg2+ over Li+ can be strongly influenced by the ring size, which was the best for the B15C5. The general accuracy of DFT is validated by comparing DFT in calculations in a polarizable continuum model with a liquid–liquid extraction in a water/dichloromethane mixture using the exemplary extraction of Li+, Na+, K+, Mg2+, and Ca2+ by aza-15-crown-5. By doing so, the DFT trends are confirmed (with the stability of the Mg2+ being overestimated while all others are underestimated), demonstrating that DFT is a valuable tool for designing optimizing CE structures to selectively complex Li+ or other metal ions. In the context of increasing global lithium demand, the results of this study can serve the further development of selective and sustainable lithium extraction from the largely unexploited resource of seawater