Aerosol Cross-Linked Crown Ether Diols Melded with Poly(vinyl alcohol) as Specialized Microfibrous Li+ Adsorbents

Crown ether (CE)-based Li+ adsorbent micro fibers (MFs) were successfully fabricated through a combined use of CE diols, electrospinning, and aerosol cross-linking. The 14- to 16-membered CEs, with varied ring subunits and cavity dimensions, have two hydroxyl groups for covalent attachments to poly(vinyl alcohol) (PVA) as the chosen matrix. The CE diols were blended with PVA and transformed into microfibers via electrospinning, a highly effective technique in minimizing CE loss during MF fabrication. Subsequent aerosol glutaraldehyde (GA) cross-linking of the electrospun CE/PVA MFs stabilized the adsorbents in water. The aerosol technique is highly effective in cross-linking the MFs at short time (5 h) with minimal volume requirement of GA solution (2.4 mL g(-1) MF). GA cross-linking alleviated CE leakage from the fibers as the CEs were securely attached with PVA through covalent CE GA PVA linkages. Three types of CE/PVA MFs were fabricated and characterized through Fourier transform infrared-attenuated total reflection, C-13 cross-polarization magic angle spinning NMR, field emission scanning electron microscope, N-2 adsorption/desorption, and universal testing machine. The MFs exhibited pseudo-second-order rate and Langmuir-type Li+ adsorption. At their saturated states, the MFs were able to use 90-99% CEs for 1:1 Li+ complexation, suggesting favorability of their microfibrous structures for CE accessibility to Lit. The MFs were highly Li+ selective in seawater. Neopentyl-bearing CE was most effective in blocking larger monovalents Na+ and K+, whereas the dibenzo CE was best in discriminating divalents Mg2+ and Ca2+. Experimental selectivity trends concur with the reaction enthalpies from density functional theory calculations, confirming the influence of CE structures and cavity dimensions in their "size-match" Li+ selectivity.11Nsciescopu

Design of lithium selective crown ethers: Synthesis, extraction and theoretical binding studies

Lithium-selective (Li+) di-hydroxy crown ethers (CEs 3a-3h) were efficiently synthesized via intermolecular cyclization of bulky bis-epoxide with 1,2-dihydroxybenzene. Bis-epoxides were produced by etherifying allyl bromides with bulky diols to afford diene intermediates, which were subsequently epoxidized. Optimized cyclization reactions were established by changing the solvent, catalyst, and reaction temperature. Complexation abilities of CEs 3a-3h with Li+ and other alkali metals (Na+, K+, Cs+) were assessed by liquid-liquid extraction in dichloromethane-water system. Among the CEs, the highest Li+/Na+ selectivities were obtained from 3d (alpha(Li/Na) = 2519) and 3e (alpha(Li/Na) = 1768). DFT calculations reveal that 3d (1.28-1.37 angstrom) and 3e (1.23-1.38 angstrom) had the closest cavity sizes with Li+ diameter (1.36 angstrom). This result affirms that the size-match selectivity of CEs with Li+ was due to the presence of bulky tetramethyl (3d) or bicyclopentyl (3e) subunits with the rigid benzo groups. Complexation with larger cations like Na+, K+ and Cs+ greatly distorted the 3d and 3e rings as indicated by the larger O-M+ distances on their bulky sides than on their benzo sides. Thus, their (3d, 3e) superior selectivities were due to their Li+ preference and unstable complexation with larger M+. Enthalpy exchange reaction mechanisms reveal the tendency of all CEs to form 2:1 CE-M+ complexes with larger cations except for 3d, which mainly forms 1:1 CE-M+ hence it is considered most suitable for Li+. The efficient synthesis of di-hydroxy CEs widens their application not only as extractants but also as solid-supported Li+ adsorbents given the amenability of their OH- groups to further functionalization. (C) 2017 Elsevier B.V. All rights reserved.11Nsciescopu

Multidentate thia-crown ethers as hyper-crosslinked macroporous adsorbent resins for the efficient Pd/Pt recovery and separation from highly acidic spent automotive catalyst leachate

Multidentate thia-crown ether (CE) diols containing different number of sulfur heteroatoms (2S-4S) were developed as ligands for Pd and Pt. Seven thia-CE diols (denoted as: 2g-2m) were synthesized at 63–93% yields through ring-opening cyclization of bis-epoxide intermediates with 1,2-benzenedithiol. Each thia-CE contains 2 –OH groups as reactive sites for adsorbent fabrication. Initial screening of thia-CE diols through liquid–liquid extraction and density functional theory (DFT) calculations reveal that bidentate (2S, 2O) dithia-CE diol 2i with cavity size Ø2i = 1.61 Å is most selective towards Pd (ØPd2+=1.56 Å) and tetradentate (4S) tetrathia-CE diol 2m (Ø2m = 1.57 Å) to Pt (ØPt2+=1.48 Å). DFT calculations indicate that size-match relationship and denticity difference dictated the coordination stability of 2i with Pd and 2m with Pt, which ultimately defined their respective selectivities. Thia-CEs 2i and 2m were subsequently fabricated as macroporous adsorbent resins (2i-X and 2m-X) via crosslinking of their bis-epoxide derivatives with ethylenediamine in porogenic PEG 400 solvent. Metal ion uptakes were Langmuir-type with high capacities (2i-X: QPd = 212 mg g−1; 2m-X: QPt = 345 mg g−1) and kinetic rates follow the pseudo-second order rate model. Metal ion uptakes are mainly due to neutral coordination with the thia-CEs (84–86%) and to some extent, due to anion complexation with ammonium groups (14–16%). Recovery of Pd by 2i-X and Pt by 2m-X can be carried out effectively and repeatedly in highly acidic feed (6 M HCl) without performance deterioration. Sequential adsorption of Pd by 2i-X and Pt by 2m-X are highly selective in the presence of base metal ions (Mg2+, Al3+, Cr3+, Mn2+, Fe3+, Ni2+, and Pb2+) making these resins ideal for the treatment of highly acidic spent auto-catalyst leachate.11Nsciescopu

Aerosol Cross-Linked Crown Ether Diols Melded with Poly(vinyl alcohol) as Specialized Microfibrous Li⁺ Adsorbents

Crown ether (CE)-based Li⁺ adsorbent microfibers (MFs) were successfully fabricated through a combined use of CE diols, electrospinning, and aerosol cross-linking. The 14- to 16-membered CEs, with varied ring subunits and cavity dimensions, have two hydroxyl groups for covalent attachments to poly­(vinyl alcohol) (PVA) as the chosen matrix. The CE diols were blended with PVA and transformed into microfibers via electrospinning, a highly effective technique in minimizing CE loss during MF fabrication. Subsequent aerosol glutaraldehyde (GA) cross-linking of the electrospun CE/PVA MFs stabilized the adsorbents in water. The aerosol technique is highly effective in cross-linking the MFs at short time (5 h) with minimal volume requirement of GA solution (2.4 mL g^–1 MF). GA cross-linking alleviated CE leakage from the fibers as the CEs were securely attached with PVA through covalent CE–GA–PVA linkages. Three types of CE/PVA MFs were fabricated and characterized through Fourier transform infrared-attenuated total reflection, ¹³C cross-polarization magic angle spinning NMR, field emission scanning electron microscope, N₂ adsorption/desorption, and universal testing machine. The MFs exhibited pseudo-second-order rate and Langmuir-type Li⁺ adsorption. At their saturated states, the MFs were able to use 90–99% CEs for 1:1 Li⁺ complexation, suggesting favorability of their microfibrous structures for CE accessibility to Li⁺. The MFs were highly Li⁺-selective in seawater. Neopentyl-bearing CE was most effective in blocking larger monovalents Na⁺ and K⁺, whereas the dibenzo CE was best in discriminating divalents Mg²⁺ and Ca²⁺. Experimental selectivity trends concur with the reaction enthalpies from density functional theory calculations, confirming the influence of CE structures and cavity dimensions in their “size-match” Li⁺ selectivity