Preparation And Performance Of Membranes From Polymer Of Intrinsic Microporosity For Lithium Selective Separation

Lithium selective separation via membrane-based technologies has drawn great attention due to its environmental friendliness,low energy consumption,and continuous operation.In this field,Li+/Mg2+ separation and 6Li+/7Li+ separation are two of the most concerned research topics.However,both pairs of the above-mentioned ions have very similar chemical and physical properties,making it difficult to separate from each other.Traditional membrane separations based on size sieving and charge repulsion effects generally lack of specific Li+recognition and preferential Li+transport,limiting the improvement of Li+/Mg2+and 6Li+/7Li+ selectivity.Supported liquid membranes and ion-sieve membranes usually process enhanced Li+selective separation capacity,but the stability of the these membranes should be improved.Therefore,it is of great significance to develop membranes with excellent selectivity and stability for efficient lithium selective separation.By combining the ion selection of crown ether with abundant free volumes for ion transport,we design a series of crown ether-based Tr?ger’s Base(TB)membranes for Li+/Mg2+ separation and 6Li+/7Li+ separation.The detailed contents of this dissertation are as follows:(1)TB microporous polymers generally have abundant free volumes due to the rigid shape-persistent structure of TB units.A typical TB polymer with high porosity,PIM-DMBP-TB,is prepared by using a commercially available diamine monomer.To prepare a functionalized microporous membrane,namely PIM-DMBP-B12C4,for Li+/Mg2+ separation,the following post-modification with 2-chloromethyl-benzo-12crown-4(B12C4-Cl)to PIM-DMBP-TB is employed.The Ⅰ-Ⅴ curves and static ion diffusion experiment indicate the prepared membrane feature a preferential Li+transport.With the 1H NMR titration experiment and ion diffusion measurements at different temperatures,it is demonstrated that B12C4 has a specific recognition of Li+,which makes Li+has a lower transmembrane transport energy barrier.The membrane has a Li+/Mg2+ selectivity of 22.41 during the diffusion dialysis with a binary Li+/Mg2+mixture.Furthermore,a Li+/Mg2+ selectivity of 11.73 with a Li+ permeation as high as 101 mmol h-1 m-2 are obtained during electrodialysis.(2)By combining dibenzo-18-crown-6(DB18C6)with TB polymer skeleton,a TB membrane,namely PIM-DB18C6-TB,with crown ether as the polymer backbone is designed for Li+/Mg2+separation.The rigid and shape-persistent TB units provide the membrane excellent mechanical properties and abundant free volumes for Li+transport.With mechanical characterization of PIM-DB18C6-TB and ion diffusion experiments,it is proved that the introduction of DB18C6 enhances the hydrophilicity and Li+selectivity of the prepared membrane.These properties promote the uncharged PIMDB18C6-TB membrane exhibits fast Li+ transport.An outstanding Li+/Mg2+selectivity of 35.80 is obtained during the static diffusion measurement with a binary Li+/Mg2+mixture.To figure out the mechanism of Li+/Mg2+ selective separation,1H NMR titration experiments of DB18C6 with different ions are carried out.It reveals that weaker interaction between Li+with DB18C6 leads to a preferentially Li+transmembrane transport,while Mg2+with large hydrated diameter and higher dehydration energy can barely recognized by DB18C6,resulting in a much lower Mg2+permeation rate.The PIM-DB18C6-TB membrane contributes to an efficient Li+/Mg2+separation in binary Li+/Mg2+ mixture,which is further demonstrated by electrodialysls.An excellent Li+/Mg2+ selectivity of 24.35 with a Li+permeation as high as 81 mmol h1m-2 are obtained,surpassing the many commercial and reported membranes.This work provides a feasible strategy for designing highly selective membranes for Li+/Mg2+ separation.(3)To effectively separate 6Li+from 7Li+,dibenzo-15-crown-5(DB15C5),whose cavity size is close to the diameter of Li+,is selected as a Li+ selective functionality.A DB15C5-based polymeric membrane is fabricated via TB polymerization,namely PIM-DB15C5-TB,for 6Li+/7Li+ separation.With mechanical characterization of PIMDB15C5-TB and ion diffusion experiments,it is demonstrated that DB15C5 enables PIM-DB15C5-TB to selectively transport 6Li+.The preferential 6Li+ transport is because 6Li+ behaves a lower transmembrane transport energy barrier through PIMDB15C5-TB,which is proved with ion diffusion measurements at different temperatures.TB units provide the membrane with excellent mechanical properties and abundant free volumes for ions transport,contributing to a fast Li+diffusion rate.With a lab-made 6Li+/7Li+ mixture,a 6Li+/7Li+ selectivity of 1.08 is collected during ion static diffusion measurement.Combing the preferential 6Li+ transport of the prepared membrane with the difference in the 6Li+and 7Li+diffusion rates under an electric field,an excellent single-stage 6Li+/7Li+ selectivity of 1.16 is obtained via electrodialysis with a 0.1 mol L-1 LiNO3 solution,surpassing the many reported materials.This work proposes a strategy for designing polymeric membranes for effcient 6Li+/7Li+ separation.