Design,Synthesis,Characterization And Applications Of Covalent Organic Frameworks

Covalent organic frameworks?COFs?are a class of crystalline porous polymers that is first reported in 2005 as a new type of porous materials by Yaghi group.Since its outstanding characters such as regular structure,low density,large specific surface area and easy surface modification,this materials have attracted wide attentions in areas such as catalysis,photoelectricity,gas adsorption and storage.Over the past decade,a large number of two-dimensional?2D?COFs with nearly eclipsed structures have been synthesized.In contrast,few threedimensional?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.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.So far,most of the examples are focusing on 2D COFs,only a few 3D COFs have been announced,mainly due to their limited availability of molecular building blocks and synthetic challenge.In addition,most of the reported 3D COFs have low endurance in the operating environments,such as moisture,solvents,acids and bases.This drawback considerably hampers 3D COFs practical application.Therefore,the preparation and application of stable 3D COFs have attracted immense attention owing to both importance and urgency.Considering the questions mentioned above,three types of crystalline porous materials were designed and synthesized successfully in this thesis.Subsequently,their structures and properties were characterized.The main contents are summarized as the following three aspects.Here we report a general strategy for generating a 3D carboxyl-functionalized COF through postsynthetic modification of a hydroxyl COF,and for the first time explore the 3D carboxyl COF for selective extraction of lanthanide ions.The obtained COF shows high crystalllinity,good chemical stability,and large specific surface area.Furthermore,the carboxyl COF displays high metal loading capacities together with excellent adsorption selectivity for Nd³⁺over Sr²⁺and Fe³⁺confirmed by the Langmuir adsorption isotherms and ideal adsorbed solution theory?IAST?calculations.In this work,we present a new strategy for 3D COFs with highly chemical stability?3D-stable-COFs?through hydrophobic engineering,in which the isopropyl groups were pre-modified on the skeletons of COFs.These obtained materials,denoted as JUC-530?JUC=Jilin University China?and JUC-531,exhibited high crystallinity,permanent pores and large surface areas?>1500 m2g-1?.More importantly,due to the fully incorporated alkyl groups that can protect the hydrolytically susceptible backbones effectively,these structures could be stable against harsh environments,such as strong acids?3 M HCl or 3 M H2SO4 for one week?,strong bases?20 M NaOH for one week?and boiling water?100oC for one month?,which is better than those of other reported 3D COFs,MOFs and molecular sieves regarding the alkali resistance.3D-stable-COFs also had promising oil/water separation performance with excellent separation efficiency?>99%?over a wide pH range.To the best of our knowledge,this study represents the first case of 3D chemically stable COFs based on the hydrophobic engineering of alkyl groups.Herein,we reported the design and synthesis of a series of Schiff base 3D COFs with low interpenetration by changing the geometric structure of ligands and utilizing its steric effect.We found that when the C center of the tetrahedron was replaced by adamantane,the interpenetration was significantly reduced and the 3D COFs with high specific surface area and open 3D pore channels were obtained.Furthermore,increase the distance between nodes,can also get a low interpenetration COF with super-large porosity and low density.In addition,the 3D COFs with low interpenetration has good thermal stability,which increase the modifiability of its pore channels.It will have an impact on the development of 3D functional materials.