(397l) Lithium Selective 14-Crown-4 Ethers: Synthesis, Polymerization and its Application for the Recovery of Lithium from Dilute Solutions

The demand to recover lithium (Li⁺) from alternative aqueous resources that contain diluted Li⁺ with abundant competing alkali metal ions (Na⁺, K⁺ and Cs⁺) has renewed research interests to develop highly selective Li⁺ sequestrants. Crown ethers (CEs) have flexible structures whose cavity sizes or metal ion affinities can be tailored to impart selectivity towards Li⁺. Among the CEs, 12-14 membered CE rings are known to form stable Li⁺ complexes in both organic and aqueous solutions, in the presence of other alkali metal ions. Specifically, dibenzo-14-crown-4 ether (DB14C4) and its derivatives are known as Li⁺ complexantsdue to their ideal cavity dimensions (1.2-1.52 Å for 4-6 coordination number with Li⁺). Despite this ideal cavity size-match relationship, complexation of DB14C4 and its derivatives with larger M⁺ remains a challenge. The sandwich type 2:1 crown ether-metal ion (CE-M⁺) complex is still remarkable in currently known 14-membered CE derivatives. This problem limits their application as Li⁺selective chelates in separation process.

A new type of lithium selective CEs containing rigid and bulky moieties were developed having both rigid and bulky subunits. The rigid aromatic groups enhance the rigidity of 14-crown-4 ether cavity while the bulky subunits provide a blocking mechanism to prevent bigger metal ions to from a complex as well as preventing formation of higher ordered sandwich type complexes. All of the CEs synthesized showed high Li⁺ distribution but 3d with bulky tetramethyl showed the highest Li⁺ selectivity in the presence of other alkali metals. Furthermore, no D_K+ and D_Cs+ were analyzed for 3d. Selectivity of α_Li/Na=3344 for 3d is among the highest in the literature.

Since the resulting 14-membered CEs are terminated with hydroxyl groups, they can be further polymerized. Like for instance via epoxy polymerization in the presence of a porogen (PEG 400) to obtain a porous hyper-crosslinked CE-epoxy resin. In particular, the CE (3d) epoxy monomer was synthesized by reacting the dihydroxy CE with allyl bromide to give CE-diene intermediate followed by epoxidation of the terminal dienes with an oxidant m-CPBA. Epoxy polymerized CE (3d) with different linear, cyclic and aromatic diamines (1,5-diaminopentane, 4,4’-methylenebis(cyclohexylamine), and 4,4’-diaminodiphenylmethane) in the presence of PEG 400 resulted in different CE epoxy polymer resins. The CE epoxy polymer resins were characterized by TGA-DSC, BET surface area analysis to determine the glass transition temperature (Tg) and porosity, and SEM to show the morphology of the synthesized polymer resin. Batch and continuous lithium extraction experiments were done to evaluate the performance of the CE-epoxy polymer resins.

This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2015R1A2A1A15055407) and by the Ministry of Education (No. 2009-0093816 and 2017R1D1A1B03028102).