Study On Preparation Of Polymer Skeleton Material And Its Iodine Adsorption Properties

With the increasing global demand for energy,nuclear energy has become indispensable due to its advantages of low carbon and high energy density.Emphasizing and developing nuclear energy can not only solve the energy problem,but also alleviate the greenhouse effect.However,the development of nuclear energy also brings some potential dangers such as the volatile radioactive pollutants caused by nuclear waste and accidents.¹²⁹I and ¹³¹I are the most harmful radionuclide,which pose a threat to the ecological environment and human health.Therefore,the research on the adsorption of radioactive iodine has been highly concerned by researchers in related fields.The solid adsorption method is currently the most important method for removing radioactive iodine due to its high removal efficiency,low cost and easy operation.At present,many studies have reported the application of porous polymer materials in the field of iodine adsorption.Among them,covalent organic framework polymer materials（COFs）have advantages over other solid adsorbents due to their low density,high specific surface area,adjustable pore properties,periodic network and excellent thermal/chemical stability and are favored.At present,studies have confirmed that COFs are outstanding iodine adsorption materials.The carbon and hydrogen in the COFs framework show a weak affinity for iodine,which limits the application of materials in the field of iodine adsorption.The introduction of electron-rich atoms into the framework facilitates the adsorption of electron-deficient iodine,thereby enhancing the interaction between the porous material and iodine,and enhancing the iodine adsorption capacity of the material.Dynamic covalent chemistry（DCC）is usually used in the process of COFs synthesis to form amorphous cross-linked polymers in the initial stage,when the reaction conditions are suitable and the reaction time is sufficient,the structure of the polymer will be rearranged after repeated bond fracture and bond formation,and then after several days of continuous“self-healing”and“error adjustment”such a“error correction”process,the porous crystal network polymer with high specific surface area would be formed eventually.The formation of covalent bonds is very important for the crystallization of COF.The traditional method of preparing COFs is solvothermal method,but this method has some problems such as harsh reaction conditions and tedious operation,which is not conducive to the mass production of COFs and restricts the widely practical application of the materials.There are many monomers for preparing COFs,and the combinations of monomers are innumerable.It will be a difficult task to properly select monomers and synthesize COFs materials which can capture iodine efficiently.How to quickly develop stable,efficient materials with ultra-high iodine capture and storage to deal with the dangers posed by nuclear power is a huge challenge.Based on the consideration of the above problems,we choose theoretical research to simulate and calculate the functional groups that have a strong interaction with iodine,and design and synthesize 9 COF materials.The main research contents are as follows:1.Theoretical research is one of the fastest and low-cost methods to screen potential iodine adsorption materials in COFs materials.This method can not only predict the potential properties of unsynthesized substances,but also has guiding significance for experimental synthesis.Due to the huge number of atoms in the COFs system,we used the tight-binding method（x TB）to simulate and calculate the binding energy between iodine and adsorbent functional groups,and screen out functional groups that can effectively capture iodine.In the process of theoretical research,we found that the iodine adsorption performance of the material is closely related to the binding energy between the material and iodine,thus supplementing the iodine adsorption capacity of the material previously proposed by Jiang et al.It is only related to the pore volume of the material.According to the theory of iodine adsorption,the binding energy between the material and iodine significantly affects the iodine adsorption capacity of the material.At the same time,according to the results of simulation calculations,COF materials with imine bonds with ultrahigh iodine adsorption capacity are predicted.2.According to the molecular simulation calculation results,we choose benzene-1,3,5-Tricarbaldehyde（TFB）and three different amine-containing monomers 1,4-diaminebenzene（DB）,benzidine（BD）and 4,4’-diamino-p-terphenyl（Td）is prepared by a reversible Schiff base reaction at room temperature to prepare a series of covalent organic polymer materials with different theoretical diameters connected by imine bonds（TFB-X COF,X=DB,BD,Td）.The obtained materials were applied to the field of iodine adsorption.The structure of the material was characterized by FT-IR and ¹³C NMR.The morphology of the material was characterized by SEM and TEM.The results of TGA and PXRD show that the materials have good stability.The materials exhibit good iodine capture ability for both gaseous iodine and solution iodine.The TFB-DB COF can capture gaseous iodine as high as 6.4 g/g（640 wt%）,and the capture capacity of iodine in solution is as high as 99.9 mg/g.The adsorbed iodine exists in the form of molecules and polyiodide ions.In addition,the iodine capture capacity of the three materials in the experiment is consistent with the theoretical prediction,which also confirms that the iodine adsorption capacity of the material is closely related to the binding energy.3.Based on the rule that the macropores can accommodate more iodine-enhancing materials’iodine capture capacity,we have chosen 1,3,5-Tris（p-formylphenyl）benzene（Tfp）and three different types of amine-containing Monomers 1,4-diaminebenzene（DB）,benzidine（BD）and 4,4’-diamino-p-terphenyl（Td）are prepared at room temperature to prepare imine bond-linked covalent organic polymers（Tfp-X COF,X=DB,BD,Td）,and the ability of the materials to capture gaseous iodine and solution iodine was investigated.The results showed that the adsorption capacity of Tfp-DB COF for gaseous iodine was the highest,reaching 5.82 g/g（582 wt%）,and the capture amount of iodine in the solution was as high as 99.9 mg/g.The iodine adsorbed in the material mainly exists in the form of iodine molecules and polyiodide anions.In addition,from the theoretical calculation results,it can be known that the iodine adsorption capacity of the three materials is closely related to the binding energy,and the binding energy significantly affects the iodine adsorption capacity of the materials.4.The reversible Schiff base reaction can make imine-bonded COF materials form crystal networks during the synthesis process.However,the reverse reaction will cause structural stability problems.Based on the above considerations,we chose 2,4,6-trihydroxylbenzene-1,3,5-tricarbaldehyde（Tpg）and three different amine-containing monomers 1,4-diaminebenzene（DB）,benzidine（BD）and 4,4’-diamino-p-terphenyl（Td）to prepare covalent organic porous materials（COF-TpgX,X=DB,BD,Td）from terphenyl,try to enhance the stability of the material through the irreversible structure conversion of enol imine and ketoenamine.COF-TpgDB and COF-Tpg BD have strong resistance to boiling water,acid and base.The adsorption and desorption performance of the material on the iodine in the solution was investigated,and it was found that the iodine removal efficiency of the material in the solution could reach more than 93%.The iodine adsorbed in the material exists in the form of molecules and polyiodide anions.Combined with the theoretical calculation results,it is also proved that the carbonyl group,amine group and benzene ring on the material skeleton can adsorb neutral iodine molecule and polyiodide anions.