Preparation Of Photoresponsive Lithium Extraction Composite Membrane And The Separation Performance Of Lithium By Electric Drive Enhancement

As the lightest metal known,lithium has controlled thermonuclear fusion characteristics and excellent electrochemical activity.It is an indispensable key raw material in clean electric energy storage,nuclear energy industry,aerospace and other fields,and has been established as one of the 24 strategic minerals in China.At present,with the rapid development of the new energy industry,our demand for lithium is increasing rapidly year by year,but the global lithium resource shows oligarchic control characteristics,our lithium raw material supply external dependence is still more than 60%.More than 80%of lithium resources in China mainly exist in the form of salt lake brine,among which the characteristics of low lithium ion concentration,high magnesium-lithium ratio and many kinds and concentrations of other ions make it difficult to separate and enrich lithium.Existing lithium extraction technologies mainly include solvent extraction method,adsorption method and membrane separation method.The development of corresponding new extractant molecules,adsorbents and membrane materials and other functional materials is the key to accelerate the green and efficient development of local lithium resources,which is of great significance to national energy security.In this paper,a series of photoresponsive spiropyran-crown ether lithium extractor molecules were designed and synthesized based on the optically induced"open-close"ring properties of spiropyran-crown ether and the selectivity of the crown ether ring to specific alkali metal ions.By mixing the spiropyran-crown ether with high polymer,a lithium extractor composite membrane with photoresponsive adsorption and desorbing properties was prepared.Furthermore,the interfacial transmission of lithium ions was enhanced by an applied electric field.Achieve high efficiency separation of magnesium lithium.The selective mechanism of spiropyran-crown ether molecules on lithium ions and the separation effect of electric field enhancement were studied by spectroscopic test and theoretical calculation.The main research conclusions are as follows:（1）Spiropyrano-crown ether molecules with different sizes of crown ether rings were synthesized from 2,3,3-trimethylindole,3-bromopropionic acid,5-nitrosalicylaldehyde,1-aza-12-crown 4-ether,1-aza-15-crown 5-ether,1-aza-18-crown 6-ether,etc.The molecular structure was characterized by ¹H NMR.It was found that three spiropyran-crown ether molecules maintained good photochromic properties after six cycles of UV/visible light,indicating that they had good photogenic open-closed cycle stability.In addition,by comparing the addition of different metal ions,spiropyran-crown ether molecules containing 1-aza-12-crown 4-ether,1-aza-15-crown 5-ether,1-aza-18-crown 6-ether have selective complexation characteristics for lithium ion,sodium ion and potassium ion,respectively.（2）The interaction of three spiropyran-crown ether molecules with different metal ions was studied by Dmol3 calculation method.Through the calculation of interaction energy and the search of the transition state of the adsorption process,the results show that the adsorption energy barrier of spiropyran-crown ether containing 1-aza-12 crown-4 for lithium ions is much lower than that of magnesium ions.Furthermore,the mean square shift,diffusion coefficient and radial distribution function of specific molecules/atoms in different systems were compared and calculated by molecular dynamics simulation method,and it was confirmed that spiropyran-crown ether containing 1-aza-12 crown-4 had strong selectivity for lithium ions.（3）Using spiropyran-crown ether,polyvinylidene fluoride（PVDF）,titanium dioxide（Ti O₂）containing 1-aza-12 crown-4 as raw materials,and N,N-dimethylformamide（DMF）as solvent,the lithium extraction composite membrane was prepared by phase conversion and membrane wiping method.The photochromic characteristics,microstructure and hydrophobicity of the membrane were studied,and the electroadsorption experiment was carried out by placing the membrane between the pubic chamber and the anode chamber as an adsorbent,and the separation performance of lithium ions under different voltages and different magnesium-lithium ratios was investigated.The results show that the composite membrane has a porous microstructure,the incorporation of Ti O₂can significantly improve the hydrophilicity of the composite membrane,and the composite membrane still maintains stable photochromic characteristics after six alternating irradiation of ultraviolet light and visible light,and still has high selectivity for lithium ions under the condition of magnesium-lithium ratio of 100,and the selectivity factor of magnesium-lithium separation is 19.21.（4）An electrode membrane material with photoresponse characteristics was prepared using spiropyran-crown ether and perfluorosulfonic acid resin（Nafion）containing 1-aza-12 crown-4 as raw materials and carbon paper as the substrate.With the membrane material as the cathode and the platinum mesh electrode as the anode,a single-chamber electroadsorption lithium extraction device was constructed in an aqueous solution containing lithium brine.In this device,the cathode and anode positions are interchanged to realize the cyclic adsorption/desorption of lithium ions.The influence of temperature,voltage,membrane area,magnesium-lithium ratio and initial lithium concentration on the performance of electroadsorption and lithium extraction was investigated,and the operating conditions were optimized.After adsorption kinetic analysis,the adsorption process was mainly physical adsorption,supplemented by chemical adsorption.This study further enhances the understanding of the molecular mechanism of lithium extraction from crown ethers,and provides a new strategy for the separation and enrichment of lithium ions in the salt lake brine system with multiple ions,which meets the new requirements of“simplification and greening”in the comprehensive utilization process of salt lake resources.