(398ap) Crown Ether Diols Aerosol Cross-Linked with Poly(vinyl alcohol) As Specialized Li+ Adsorbent Nanofibers
Lithium-specific crown ether (CE)-based adsorbent nanofibers (NF) were successfully prepared through strategic use of CE diols, electrospinning and aerosol cross-linking. Each diol contained four ethereal oxygens for Li+ complexation, a benzo group for a rigidified ring, and two hydroxyl groups for covalent attachments to poly(vinyl alcohol) (PVA) as the chosen matrix. The CE diol/PVA blends were electrospun into NFs to obtain structures that are favorable for Li+ interaction. Electrospinning also eliminated CE wastage during CE/PVA NF fabrication. As a post-electrospinning treatment, aerosol glutaraldehyde (GA) acetalization not only stabilized the PVA in water (insoluble) but also effectively linked the CEs to PVA by forming CE-GA-PVA bridges. The devised cross-linking method preserved the NF structure, required short reaction time (5 h) and used low cross-linker volume (2.4 mL g-1 NF). From these techniques, three types of CE/PVA NFs were fabricated. The CEs slightly differ in ring size based on DFT calculations and in one-side ring sub-units (i.e. R=benzyl, neopentyl, butyl). The effectiveness of the fabrication strategy was confirmed through FTIR-ATR, 13C CP-MAS NMR, FE-SEM, N2 adsorption/desorption, and UTM. The NFs exhibited pseudo-second order rate and Langmuir-type of Li+ adsorption. Their nearly ideal performances reveal 90-99% CE utilization through 1:1 Li+ complexation. The NFs were highly selective to Li+ in seawater; neopentyl bearing CE was most effective in blocking larger monovalents like Na+ and K+ while the dibenzo CE was best in discriminating divalents like Mg2+ and Ca2+. Experimental selectivity trends concur with the reaction enthalpies from DFT calculations, confirming the influence of CE structures and cavity dimensions in their "size-match" Li+ selectivity. The key advantage of the fabrication strategy is its versatility for wide application. By simply replacing the CE diols, numerous types of CE-based NF adsorbents can be developed for the recovery of precious or highly toxic metals.
This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2016R1A2B1009221 and 2017R1A2B2002109) and the Ministry of Education (2009-0093816).