Engineering Linkage As Functional Moiety Into Covalent Organic Framework For Adsorption Of Pollutants

Water pollution due to natural and anthropogenic activities has become a global problem,and a large number of organic pollutants,when they are discharged into the natural environment such as water or soil through point and non-point sources,may not only continuously accumulate in the natural media such as air,soil,and water bodies,causing serious environmental safety problems,but also may be transferred and enriched in living organisms,entering the human body through the food chain.It seriously threatens food safety and human health.Therefore,the development of highly efficient adsorbents for the removal of environmental pollutants is of great significance for protecting the environment and improving human living standards.Covalent organic frameworks（COFs）have emerged as a novel class of adsorbents due to their high crystallinity,Ordered structure,functional modifiability,structural tunability and relatively high chemical and thermal stability as porous crystalline materials.Functionalization is an important means to improve the adsorption properties of COFs,but existing functional strategies tend to occupy the pore channels,causing functional site shielding problems.Therefore,this paper intends to use the linkage bond generated after the reaction of COF monomers as a functional moiety to prepare COFs to solve the problem,and to further explore their potential as adsorbents for pollutant removal in the environment.The main results are as follows:（1）Proposed a functionalization strategy that integrating linkage bonds and functional groups to prepare structurally ordered crystalline ethylenediimide-based COFs.Tetrakis（4-aminobenzene）porphyrin monomers with pyridine-2,5-diformaldehyde as reaction monomers and o-dichlorobenzene and ethanol as solvents were selected and synthesized at 120℃for four days.The prepared COF is a periodically arranged crystalline porous polymer with nanosheet morphology,specific surface area,average pore size and pore capacity of 864.579 m² g^-1,1.20nm,1.38 cm³ g^-1,which is highly chemically stable in solvents such as methanol,acetonitrile（ACN）and tetrahydrofuran（THF）,and has excellent thermal stability with only 20%weight loss at 800℃,thus laying the foundation for its subsequent application as an adsorbent for complex systems.To verify the generality of the functional strategy with linkage bond and functional group,thiourea-based COFs were further prepared with the above strategy.Thiourea-based COF has a porous crystal structure with specific surface area and pore volume of 1050m² g^-1 and 3.0 cm³ g^-1,respectively,and is structurally stable in ethanol,acetone,0.1 mol L^-1Na OH and HCl solutions.（2）Using ethylenediimide-based COFs as adsorbents,the rapid and efficient adsorption of2,6-dichlorophenol and p-nitrophenol could be achieved within 1 h and 30 min,respectively.By examining the adsorption kinetics and thermodynamics of phenolics in water,the maximum adsorption adsorption capacities reached 309.6 and 229.9 mg g^-1.The 2,6-dichlorophenol and p-nitrophenol adsorbed on the ethylenediimide-based COFs could be desorped with methanol,ethanol,and acetonitrile,and the desorption efficiency reached more than 90%.After being used four times,the 2,6-dichlorophenol still maintained 90%adsorption effect,while the p-nitrophenol adsorption effect was almost unchanged.The ethylenediimide-based COFs appeared stable at p H 3-10 and aqueous phenol solutions containing 1 M Na Cl or Mg SO₄,with removal efficiencies approaching 100%in spiked real water samples.The prepared thiourea-based COF exhibited ultrafast adsorption kinetics toward Hg（II）with 10 s to complete Hg（II）adsorption from aqueous solution and the maximum adsorption capacity of 561 mg g^-1.The thiourea-based COF showed a strong affinity for Hg（II）(K_d,1.69×10⁶ m L g^-1)and achieved99.9%Hg（II）removal in aqueous solutions containing many competing ions such as Cd,Cr and Zn.