Porphyrin-Based Covalent Organic Frameworks For Efficient Capture Of Iodine And Methyl Iodide

Nuclear energy plays an important role in meeting energy demands and addressing environmental pollution issues.As one of the harmful radioactive wastes generated during the operation of nuclear power plants,the effective capture of radioiodine(129I and 131I)is of great significance for environmental protection and human health.Covalent organic frameworks(COFs)are promising iodine adsorbent materials,but current research mainly focuses on the capture of molecular iodine.The capture of organic iodides(e.g.,methyl iodide,ethyl iodide)and water-phase iodide ions is still in the development stage.It is necessary to develop multifunctional adsorbents that can capture various forms of radioiodine in different systems.Therefore,a series of porphyrin-based covalent organic framework materials(TAPP-COFs)were synthesized based on highly conjugated and nitrogen-rich porphyrin units,and were creatively applied to the adsorption experiments of iodine and methyl iodide under different conditions.The interaction mechanisms between TAPP-COFs and iodine/methyl iodide were further explored by combining the experimental results and spectroscopic analyses.The specific research contents are as follows:(1)Four covalent organic frameworks(TAPP-COF1,TAPP-COF2,TAPP-COF3,TAPP-COF4)were synthesized via condensation of 5,10,15,20-tetrakis(4aminophenyl)porphyrin(TAPP)with terephthalaldehyde(L1),4,4’biphenyldicarbaldehyde(L2),pyridine-2,5-dicarbaldehyde(L3)and[3,3’-bipyridine]6,6’-dicarbaldehyde(L4),respectively.The difference in linear monomer length and pyridine quantity(denoted as XLi,XL1-XL2,XL3=1,XL4=-2)endows TAPP-COFs with different porosity and functional group content.The structure,morphology,and physicochemical properties of TAPP-COFs were studied by a series of characterization tests.Experiments and theoretical simulations indicated that TAPP-COFs have AA stacking layered structure.Good thermal stability and chemical stability make it have the potential to capture radioiodine and methyl iodide in different systems.(2)The adsorption performances of TAPP-COFs for iodine/methyl iodide were systematically studied.Experiments showed that TAPP-COFs have excellent adsorption capacities for gaseous iodine/methyl iodide.At 75℃,the equilibrium iodine adsorption capacities of TAPP-COF1,TAPP-COF2,TAPP-COF3,and TAPP-COF4 were 4.97,5.08,5.29,and 5,8 g/g,respectively,and the equilibrium methyl iodide adsorption capacities were 1.57,1.4,1.9,and 2.0 g/g,respectively.The adsorption kinetics of iodine/methyl iodide onto TAPP-COFs are more consistent with the pseudosecond-order kinetic model,illustrating the interactions between TAPP-COFs and iodine/methyl iodide were dominated by chemisorption mechanism.In addition,the adsorption behaviors of iodine onto TAPP-COF3 and TAPP-COF4 in different solution systems(iodine cyclohexane,iodine(I2)aqueous,and iodide ions(NaI3)aqueous solutions)were also investigated.It was found that the influences of solvent molecules resulted in TAPP-COF3 exhibiting relatively excellent iodine adsorption performances.The penetration experiments of low concentration iodine aqueous solutions showed TAPP-COF3 has good dynamic iodine adsorption ability.What’s more,the stable iodine adsorption capacity and high selectivity of TAPP-COF3 in iodide ions aqueous solutions were also revealed by the adsorption kinetics experiments under different conditions(unregulated pH,pH=2,groundwater)and ion competition experiments.(3)The interaction mechanisms between TAPP-COFs and iodine/methyl iodide were proposed as follows.i)Charge-transfer between electrophilic iodine molecules and electric-rich N sites(i.e.,imine N,aza-states N(=N-)in porphyrin and pyridine N)/π-conjugated systems transformed them into polyiodide anion[Ix-]adsorbed in the material framework.ii)Methylation between methyl iodide and electric-rich N active sites formed onium salts.To summarize,the new applications of porphyrin-based COFs in the field of iodine/methyl iodide capture were explored in this work.The interaction mechanisms between TAPP-COFs and iodine/methyl iodide provide valuable reference for the design and development of advanced and efficient multifunctional radioiodine adsorbents.