Postsynthetic Functionalization Of Three-Dimensional Covalent Organic Framework For Selective Extraction Of Lanthanide Ions

Covalent organic frameworks?COFs?is a new class of crystalline porous polymers based on the precise integration of organic building blocks into periodic structures.Because of their regular pore structure,high surface area and tunable chemistry,the COF materials are explored in various fields such as energy storage,optoelectronics,catalysis,drug delivery and a number of others.Over the past decade,a large number of two-dimensional?2D?COFs with nearly eclipsed structures have been synthesized.In contrast,few three-dimensional?3D?COFs have been obtained,and the functionalization of 3D COFs still remains largely unexplored.Functionalization of the pores of 3D COFs is a straightforward way to predetermine the properties and provide structurally and chemically precise platforms for various applications.However,the introduction of functional groups in 3D COFs is difficult due to the absence of reactive groups,such as hydroxyls,in almost all 3DCOFs reported to date.Thus,in contrast to zeolites and metal-organic frameworks?MOFs?where the postsynthetic modification is readily used,the functionalization of 3D COFs is considered to be a great challenge.Herein,we report a general strategy for preparation of 3D carboxyl-functionalized COF through postsynthetic modification of a hydroxyl COF.Our strategy for preparation of a carboxyl-functionalized COF,denoted3D-COOH-COF,isbasedon theringopeningreactionapproach.Tetra?4-formylphenyl?methane?TFPM?was designed as a tetrahedral building unit,and 3,3?-dihydroxybenzidine?DHBD?was chosen as an ideal linear linker comprising hydroxyl groups.The condensation of TFPM and DHBD produces a 3D hydroxyl COF,3D-OH-COF.Subsequently,the hydroxyl groups in 3D-OH-COF are reacted with succinic anhydride?SA?resulting in a ring opening and formation of3D-COOH-COF.The obtained 3D COF shows good crystalllinity,high chemical stability,and large specific surface areas.More importantly,the carboxyl-functionalized 3D-COOH-COF showed exceptional lantanide selectivity(Nd³⁺)based on the Langmuir adsorption isotherms and IAST calculations.XPS and solid-state NMR investigations verified the strong and selective interaction between Nd³⁺ions and carboxyl groups in 3D-COOH-COF.Recyclability experiments demonstrated that multiple adsorption/stripping cycles could be performed with minimal degradation of the adsorption capacity of3D-COOH-COF.To the best of our knowledge,this study is the first example of the application of 3D COF material for selective extraction of metal ions.In summary,we report a strategy for functionalizing 3D COFs through postsynthetic modification.This study not only develops a general method for a facile and efficient functionalization of 3D COFs,but also promotes functionalized COF materials into an excellent scaffold for environmental related applications.