Synthesis Of Pillar[5]arene-based Heterocyclic N-donor Extractants And Their Separation Performance Towards Lanthanides And Actinides

As a clean and high-storage candidate,nuclear energy is one of the most promising energy sources in the future.However,the use of nuclear energy will inevitably be accompanied by a large amount of nuclear waste,which poses huge hidden dangers to environmental safety.Neptunium,americium,and curium are the main sources of heat load and radiotoxicity in nuclear waste,thus their effective partitioning and transmutation are key steps in nuclear waste treatment.However,the physical and chemical properties of lanthanides（Lns）and actinides（Ans）are too similar to separate them from each other.A large number of theoretical studies suggested that the covalency of actinides and ligands when forming coordination bonds is higher than that of lanthanides,so soft donors such as nitrogen and sulfur often exhibit good selectivity toword actinides.Compared to sulfur-containing ligands,most nitrogen ligands only contain C,H,O,and N atoms.After being used and completely incinerated,no solid residue is produced,which causes less secondary pollution to the environment and,therefore,nitrogen ligands are considered to be more environmental friendly.Among many nitrogen-containing ligands,the nitrogen atoms in the aromatic nitrogen heterocycles show strong coordination ability to metals because of the sp²hybridized nature,therefore,the aromatic nitrogen heterocyclic ligands have been extensively studied in the field of metal coordination chemistry.Pillar[5]arenes are a class of macrocyclic compounds with symmetrical and easily modified structures and rigid frameworks.They possess broad applications in many fields such as supramolecular chemistry,drug delivery,and metal ion separation and detection.Our previouse works have demonstrated that when pillar[5]arene is used as a pre-organized platform,it can endow several ligands with stronger synergistic effect in the same plane,and thus effectively improve their extraction ability,selectivity,and irradiation stability.However,no breakthrough that utilizing pillar[5]arene ligands for selective extraction of long-lived MAs has been achieved before this thesis.In this dissertation,a variety of pillar[5]arene-based N donor ligands are successfully designed and synthesized for the first time.Their extraction and complexing behavior toward Lns and MAs is investigated.In addition,the current work of our group is mainly based on the solvent extraction method,while the use of pillar[5]arene-based adsorbents for the adsorption and separation of metal ions keeps untouched.Therefore,the work in this thesis also prepared a novel pillar[5]arene-DGA impregnated Amberlite-XAD7 resin for selective lanthanide adsorption.The main contents of this thesis are listed below:1.For the first time,we designed and synthesized a series of pillar[5]arene-based picolinamide ligands with different spacers substituents for Am（Ⅲ）/Eu（Ⅲ）separation.Solvent extraction experiments that using octyloxy o-nitrobenzene（NPOE）/1,2-dichloroethane（DCE）7/3（v/v）as the diluent and 8,9,12,8’,9’,12’-hexabromo-1,2-（dicarboboron）cobalt-cesium（Br-Cosan）as a synergist showed that 1 m M of these ligands can highly efficiently separate Am（Ⅲ）from Eu（Ⅲ）at the acidity of p H 3.0(SF_Am/Eu>11).Weakening the basicity of the pyridine nitrogen by introducing an electron-withdrawing group（-Cl）at the para position can significantly improve the extraction capacity of the ligand.The distribution ratio to Am(D_Am)with an electron-withdrawing group is 300 times higher than that with an electron-donating group（-OMe）.Slope analysis,NMR titration,IR spectroscopy,X-ray absorption fine structure spectroscopy（EXAFS）and density functional theory（DFT）calculations showed that both sides of the pillar[5]arene-picolinamide can bind a metal ion through the nitrogen atom in the pyridine group and the oxygen atom in the amide group.In addition,the extraction performance of these ligands is not significantly reduced after gamma irradiation at a dose of up to 250 k Gy,which proves their perfect irradiation stability.2.Ionic liquids（ILs）are a kind of liquid salts containing organic anions or organic cations.Their low toxicity,low volatility,and non-flammability endow them with many potential applications in the field of metal extraction.Reported studies have shown that in extraction systems with ionic liquids as the diluent,when metal ions are extracted into the organic phase by the extractant,the cations of the ionic liquids are often exchanged into the aqueous phase in order to maintain the charge balance,which leads to significant improvement on the extraction performance.This work studied the extraction performance of three pillar[5]arene-base picolinamides containing different substituents（methoxy group,hydrogen atom,chlorine group）in ionic liquid.Solvent extraction experiments showed that when ionic liquids are used as the diluent,the ligand with electron-donating group（methoxy group）exhibits the best extraction and separation performance.On the contrary,the ligand with an electron withdrawing group（chlorine group）showed the worst performance.These results are totally contrary to those extraction results by pillar[5]arene-picolinamides in NPOE/DCE.Although the highest separation factors obtained here are similar to that in molecular solvent(20>SF_Am/Eu>10),the amount of extractant is significantly reduced from 1m M to 0.2 m M.In addition,comparative experiments showed that the synergist Br-Cosan is not necessary in pillar[5]arene-picolinamide-IL system.This may be caused by the large amount of anions in IL,and thus the synergist Br-Cosan that works as anion resource is no longer needed.In this work,the reagent consumption is greatly reduced and the use of Br-Cosan is avoided,which makes pillar[5]arene more suitable for nuclear waste treatment.3.Since the nitrogen atoms in nitrogen heterocycle extractants are easy to be protonated,the coordination and extraction ability of these N donor ligands is greatly reduced under high acidity.Therefore,most nitrogen heterocycle extractants are not applicable under highly acidic conditions.Although pillar[5]arene-based picolinamide ligands have achieved highly efficient Am（Ⅲ）/Eu（Ⅲ）extraction and separation at p H3.0,the extraction performance is obviously effected by acidity.In order to solve the problem above,this work pre-organized pyridine triazole（Py Tri）onto the pillar[5]arene skeleton（P[5]A-Py Tri）for the first time,so that multiple Py Tri groups can work together in the same plane,which transfers hydrophobic Py Tri of extremely poor performance into an excellent ligand with high selectivity for Am（Ⅲ）/Eu（Ⅲ）separation at high acidity（>1 M HNO₃）.Solvent extraction results showed that the P[5]A-Py Tri gives a distribution ratio of 12.9 to Am（Ⅲ）and only 0.07 to Eu（Ⅲ）under the condition of 1 M HNO₃.The SF_Am/Eu value is as high as 172,which is much higher than pillar[5]arene extractant(SF_Am/Eu=19).In addition,at 2 M HNO₃,the distribution ratio of P[5]A-Py Tri to Am（Ⅲ）is 1.9,which means that 66%of Am（Ⅲ）is extracted into organic phase,while the distribution ratio to Eu（Ⅲ）is only 0.002（0.2%Eu（Ⅲ）is extracted）,corresponding to a separation factor as high as 960.It is worth noting that the distribution ratio of Py Tri to Am（Ⅲ）under the condition of 1 M HNO₃ is only0.003,indicating that the pre-organization effect of pillar[5]arene platform can greatly improve the extraction.These results not only demonstrated a highly efficient extractant（P[5]A-Py Tri）for MAs/Lns separation,but also provided an efficient method for the design and optimization of other type of extractants by pre-organization.4.Task specific ionic liquid（TSIL）,prepared by introducing additional functional groups into the anions or cations of the ionic liquid,usually exhibits many charming properties.The main advantage of TSIL for solvent extraction is that they can be used as both diluents and extractants.TSIL combines the advantages of low toxicity,non-flammability,and high boiling point of ionic liquid,and the extraction ability and selectivity of chelating groups.In this work,a novel pyridine triazole functional task specific ionic liquid（TSIL-Py Tri）was designed and synthesized for the extraction and separation of MAs from Lns under acidic conditions.The extraction results showed that the as-prepared TSIL-Py Tri exhibits the best Am（Ⅲ）/Eu（Ⅲ）separation performance under the condition of 0.01 M HNO₃,with the separation factor being as high as 210.Slope analysis showed that the stoichiometric ratio of the metal-ligand complex during the extraction process is 1:3（M/L）,suggesting that the metal coordination environment is generally similar to that observed in P[5]A-Py Tri.5.As a highly acid-resistant oxygen donor ligand,pillar[5]arene-basd diglycolamide ligand（P[5]A-DGA）exhibits good selectivity for lanthanides.In this part,P[5]A-DGA is impregnated into Amberlite XAD-7 resin for lanthanide adsorption.The control experiment showed that the blank Amberlite XAD-7 resin has no obvious adsorption ability towards the tested lanthanides,while the P[5]A-DGA impregnated resin can separate the lanthanide metals from alkali metals and alkaline earth metals with high selectivity.This material presents good adsorption and separation performance at 0.1-3 M HNO₃.In addition,quantitative adsorption of middle and heavy lanthanides can be achieved.The kinetics experiments suggested that the adsorption process conforms to the pseudo-secondary adsorption equation,indicating that the driving force of adsorption is chemical complexation rather than physical adsorption.Results of isothermal adsorption experiments showed that the maximum adsorption capacity of Eu（Ⅲ）is 33 mg/g.In the structure of the P[5]A-DGA-Eu（Ⅲ）complex obtained by DFT calculation,each Eu（Ⅲ）ion is coordinated with three DGA arms.These results above indicated that the P[5]A-DGA impregnated resin are expected to be used for the separation of lanthanides under a wide range of acidity in spent fuel treatment.