(446m) Electrospun Poly(Glycidyl Methacrylate)-Based Crown Ether Nanofibers As Lithium Ion Adsorbent

Crown ethers (CE), especifically 12 to 14-membered CE rings, are most selective towards Li⁺ due to their matching cavity size (1.2-1.5 Ã?) with the ionic diameter of Li⁺(1.36 Ã?) [1]. However, CEs are often integrated in liquidâ??liquid extraction or liquid membrane systems which are tedious to perform and have low to moderate efficiency [2]. Thus, a more effective and convenient immobilization technique is by incorporating the CEs in solid supports.

Herein, electrospun poly(glycidyl methacrylate)-based crown ether (PGMA-CE) nanofibers (NF) were prepared, characterized and tested for lithium ion (Li⁺) adsorption. The CEs were covalently attached on the PGMA NFs via â??clickâ? chemistry. Poly(glycidyl methacrylate) (PGMA) is an excellent candidate as CE support since it possesses the pendent epoxide groups, which are excellent sites for CE attachment. Glycidyl methacrylate (GMA) monomers were polymerized via radical polymerization to synthesize the PGMA. The PGMA polymer was then electrospun to produce the PGMA NFs. After which, PGMA NFs was azidated to introduce the azide moieties in the PGMA NFs via the epoxide ring-opening mechanism (PGMA-N₃). Meanwhile, the CE hydroxydibenzo-14-crown-4 ether (HDB14C4) was modified to contain a terminal alkyne group. HDB14C4 was esterified with propiolic acid (PA) using dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) as coupling reagent and catalyst, respectively, to synthesize dibenzo-14-crown-4 propiolate (DB14C4P). Finally, the CE-â??clickedâ? PGMA NFs were produced via alkyne-azide cycloaddition (CuAAC), involving the DB14C4P and PGMA-N₃. The success of each synthesis step was confirmed through Fourier transform infrared (FTIR) spectroscopy, thermogravimetric and elemental analyses. Meanwhile, the successful modification of CE was determined using ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy. Different types of prepared nanofibers were characterized using scanning electron microscopy (SEM), ¹³C and ¹⁵N solid-state nuclear magnetic resonance (SS-NMR) spectroscopy and universal testing machine (UTM). The performance of PGMA-CE NFs as Li⁺ adsorbent was investigated for selective Li⁺ capture. Batch adsorption studies at varied Li⁺ concentrations (7~70 mg/L), pH=11 and solid/liquid ratio of 0.625 mL/mg reveal its Langmuir-type behavior and pseudo-second-order rate of Li⁺ uptake. The maximum Li⁺ adsorption capacity (q_m) was determined to be 12.3 mg g^-1, which is the highest among other organic-based Li⁺ adsorbents reported in literature. Adsorption loss after five adsorption-desorption cycles was negligible, which suggests the long-term stability of PGMA-CE NFs. In the presence of competing cations, the PGMA-CE NF was highly selective towards Li⁺ following the uptake sequence: Li⁺ > Na⁺ > K⁺ > Mg²⁺ > Ca²⁺ > Sr²⁺. Overall results indicate the potential use and reusability of PGMA-CE NFs as a highly effective and high capacity Li⁺adsorbent.

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2009-0093816) and Ministry of Science, ICT and future Planning (2015R1A2A1A15055407).

References:

1. C.J. Pedersen, Cyclic polyethers and their complexes with metal salts, J. Am. Chem. Soc., 89 (1967) 7017â??7036

2. J.G. Huddleston, H.D. Willauer, R.P. Swatloski, A.E. Visser, R.D. Rogers, Room temperature ionic liquids as novel media for â??cleanâ?? liquidâ??liquid extraction, Chem. Commun., 16 (1998) 1765â??1766