Crown Ether Functionalized Zirconium-based And Chromium-based MOFs:synthesis And Separation Performance On Lithium Isotopes

⁶Li and ⁷Li are two stable isotopes of lithium in nature,with abundances of 7.5%and 92.5%,respectively.Their nuclear reactivity is completely different.⁶Li is an essential material for nuclear fusion reaction,which can produce tritium by neutron irradiation,and its abundance is required to be greater than 30%.While high-purity⁷Li（>99.995%）is usually used as p H regulator in pressurized water reactor of nuclear power plant and coolant in thorium molten salt reactor.The current separation method of lithium isotopes in industry is mainly lithium amalgam.Although this method has the advantages of rapid mass transfer and high separation factor,a large number of highly toxic substances containing mercury will be produced in the separation process,which will cause serious environmental problems.Therefore,it is of great research significance to develop new and environmentally friendly methods for the separation and purification of lithium isotopes.Due to their unique cavity structures and size effect,the seperation factor of ⁶Li/⁷Li by crown ethers is comparable to that of lithium amalgam method,and it can be grafted onto the polymers,fibers or silica solid materials.Metal-organic frameworks（MOFs）have the advantages of easy cutting,easy functionalization and high specific surface area,as well as excellent thermal and chemical stability,which makes it possible for MOFs to support crown ether through post-synthesis modification.It can not only improve the adsorption capacity of MOFs for lithium ions,but also helpful to improve the separation factor of lithium isotopes.In this paper,based on two stable MOFs,new MOFs functionalized with crown ether were synthesized by post-synthesis modification,and their separation properties of lithium isotopes were studied,specific work is as follows:（1）Zr-SXU-12 with high stability was synthesized by solvothermal method.Using it as the precursor,the target material Zr-SXU-12-N15C5 was obtained after continuous synthesis and modification with trimethyl bromide silane and aza-15-crown ether-5（N15C5）.IR,NMR and gas adsorption characterization have proved that aza-15-crown ether-5 was successfully introduced into the pore species of Zr-SXU-12.The results of the lithium ion adsorption performance test show that the maximum adsorption capacity of the material for lithium ion was 50 mg·g^-1after functionalization,which was much higher than that before functionalization.The separation factors of lithium isotopes for Zr-SXU-12 and Zr-SXU-12-N15C5 are1.018 and 1.040,respectively.This indicates that Zr-SXU-12-N15C5 can be used as a material for the adsorption and separation of lithium ions.（2）Cr-SXU-3-N15C5 was synthesized and functionalized by two steps using Cr-SXU-3 as the precursor.After functionalization,the frame structure of the modified material is still retained,but the specific surface area decreases.The maximum adsorption capacity of lithium ion is 62.56 mg·g^-1,and the maximum separation factors of lithium isotopes is 1.061.After five cycles of adsorption in the aqueous solution of lithium acetate with the concentration of lithium ion is 10 mg·L^-1,the adsorption of Li⁺can still reach 25 mg·g-1.In addition,the framework stability of the material is stronger than that of Zr-SXU-12-N15C5.Therefore,the functionalzed materials Cr-SXU-3-N15C5 has the potential to be used as Li⁺absorption separation materials.