The Preparation Of A Polyimide Membrane Containing Dibenzo-14-crown-4 In The Main Chain And Its Lithium Ion Selective Separation Performance

The development of polymer membrane materials with high selective separation performance has important significance and application value for the extraction and efficient utilization of lithium resources in salt lakes.The aim of this paper is to prepare dibenzo-14-crown-4-ether-containing polyimide?PI?by condensation polymerization using diaminobenzo-14-crown-4 and diacid anhydride as monomers.Simultaneously,the non-solvent induced phase separation?NIPS?method was used to fabricate porous polymer membranes using PI as raw material.The performance of PI polymer and porous membrane for lithium ionic and lithium isotope selective separation was studied by dynamic adsorption experiment.First,dibenzo-14-crown-4?DB14C4?was synthesized by template method.Dinitrodibenzo-14-crown-4?DNB14C4?was obtained through substitution reaction,and diaminodibenzo-14-crown-4?DAB14C4?was produced by the reduction of DNB14C4.Finally,Poly?DAB14C4-6FDA?with the crown ether immobilization amount of?I_A?1.36 mmol/g and M_n=63 k Da was synthesized through condensation polymerization with 4,4'-?hexafluoro-isopropylidene?diphthalic anhydride?6FDA?.The lithium ions adsorption and lithium isotope separation performance of three crown ethers DB14C4,DNB14C4,DAB14C4 and Poly?DAB14C4-6FDA?were studied by liquid-liquid extraction and liquid-solid extraction,respectively.The results showed that the maximum adsorption capacity of lithium ion was up to 0.78 mg/g of DAB14C4containing electron-donating group.The maximum lithium isotope separation factor of Poly?DAB14C4-6FDA?was 1.024,indicating an enrichment of ⁶Li.The Poly?DAB14C4-6FDA?polymer as raw material,N,N-dimethylformamide?DMF?as the solvent was employed to prepare a casting solution.The Poly?DAB14C4-6FDA?porous membrane was prepared by NIPS method using DI water as a coagulation solution and polyvinylpyrrolidone?PVP-K30?as additive.It was found that the porous membrane obtained with the average pore diameter of 18.4 nm,the porosity of 80.5%,and the mechanical strength of 6.16 MPa displayed a finger-like pore structure,when the polymer concentration was 16 wt.%.The pure water permance was580 LMH/bar.In addition,the concentration of PVP-K30 in the casting solution was up to 8%,the resultant porous membrane with the average pore size of 11.8 nm,the porosity of 73.4%and the mechanical strength of 8.75 MPa showed a completely sponge-like pore structure.The pure water permance was 305 LMH/bar.The dynamic adsorption method was used to further explore its performance of porous membrane for lithium ionic and lithium isotope selective separation.The results showed that the Li⁺adsorption equilibrium times were 240 min for the porous membrane obtained from the PI/DMF casting solution.And the Li⁺adsorption kinetics followed the pseudo-secondary adsorption equation and its adsorption isotherms data fitted the Langmuir model,indicating a monolayer chemical adsorption.The maximum adsorption capacity of Li⁺for Poly?DAB14C4-6FDA?porous membrane prepared by the PI/DMF casting membrane solution system was 34.05 mg/g in the Li I aqueous solution of 500 mg/L.The selective separation factor of Li⁺to Na⁺,K⁺,Mg²⁺and Ca²⁺in a mixture aqueous solution of LiCl,NaCl,KCl,MgCl₂ and CaCl₂ at the same concentration of 50 mg/L was 45.6,48.3,23.5,and 41.2,respectively.Based on the density functional theory?DFT?,the Gaussian 09 program?DFT-B3LYP/6-311G+?d,p??was used to simulate and calculate the adsorption energy of Poly?DAB14C4-6FDA?polymer when the membrane was complexed with Li⁺/Mg²⁺and the binding enthalpy when the membrane was complexed with ⁶Li/⁷Li.Results showed that the PI porous membrane exhibited the lowest energy to coordinate with Li⁺and ⁶Li and form the strongest complexation.The DFT simulation is consistent with the experimental results.