Preparation Of Nitrogen-rich Covalent Organic Frameworks And Study On Their Iodine Adsorption Properties

Currently,nuclear energy as a low carbon energy source is carrying about 10%of the world’s electricity.However,radioactive gaseous fission products such as ¹²⁹I,¹³¹I,¹²⁷Xe and ⁸⁵Kr from nuclear fission waste or severe nuclear accidents can have a very negative impact on human health under long-term high radiation conditions.Radioactive iodine,one of the main products of nuclear fission,is particularly hazardous to the environment due to its high volatility and radioactivity.Among the radioisotopes of iodine,¹³¹I,despite its relatively short half-life（about eight days）,continues to damage the thyroid and interfere with the body’s metabolic processes once it enters the body,so it should be captured immediately after release.Therefore it is urgently necessary to develop new safe,efficient and economical materials for the adsorption and storage of iodine residues.As a class of crystalline porous polymers,covalent organic frameworks（COFs）have become potential candidates for iodine adsorption carriers due to their high specific surface area,ordered pore channels,good chemical stability,specific pore environment and easily functionalised characteristics,which facilitate the design of materials with effective iodine capture active sites.Based on this,we have synthesised a series of nitrogen-containing COFs materials and applied them to the field of iodine adsorption,as follows:1.Three covalent organic frameworks containing different types of electron-rich nitrogen（TAPT-BDA,TAPB-BDA,TAPB-BD）were synthesized and tested for iodine adsorption by the Schiff base reaction.The composition and structure of the materials were investigated systematically by various characterisation methods such as powder X-ray diffraction and infrared spectroscopy.The iodine adsorption experiments showed that TAPT-BDA,TAPB-BDA and TAPB-BD had good iodine adsorption performance,with saturation values of 5.43,5.05 and 5.21 g g^-1,whereas the post-adsorption stationary iodine tests and recycling experiments showed that the three COFs were mainly chemically adsorbed and had great recycling performance.Further studies have shown that the prepared COF materials also have a certain adsorption capacity for iodine monomers in liquids.2.Based on the previous work on the iodine adsorption performance of nitrogen-containing COFs,in this chapter we have further developed and enhanced the nitrogen content of the COFs backbone and successfully designed and synthesised the COF material（TPTA-BD）for the efficient capture of gaseous iodine and methyl iodide.Static iodine vapour adsorption studies on TPTA-BD have shown that TPTA-BD is an excellent iodine adsorption material with a saturation value of 7.25 g g^-1 for vapour iodine.The ability to capture gaseous methyl iodide,a derivative of iodine,was also investigated and the saturation value of methyl iodide for TPTA-BD was shown to be0.76 g g^-1.The adsorption mechanism of TPTA-BD is then discussed and the results indicate that the electron-rich group nitrogen in the material plays an important role in the overall adsorption process.