Synthesis And Applications Properties Study Of Hollow Covalent Organic Frameworks Spheres

Covalent organic frameworks(COFs),as an emerging vital class of organic porous material with crystalline morphology,are connected by strong covalent bonds.Both 2D and 3D covalent organic frame materials possess open and regular pore structure,high specific surface area and are easy to designed and functionalized.Based on this,since the first coverage of COFs by the Yaghi gruop in 2005,the material has attracted wide attention from global scholars.After more than ten years of research and development,COFs materials have been successfully applied in the fields of photoelectric devices,biochemical induction,drug delivery,gas separation,catalysis and adsorption,exhibiting better potential applications than other porous materials.However,the applications of COFs materials in these applications are all based on the powder morphology,leading to the failure to fully utilize the properties of regular channels of COFs materials.The purpose of this study is to enhance the interaction and selectivity of COFs pores to guest molecules by adjusting the morphology of COFs,and further apply them to drug loading to enhance the controlled release effect and heterogeneous catalysis to improve the selectivity and stability of catalyst.The main contents include the following three aspects:(1)Synthesis,regulation and characterizationof enol-keto tautomerism COFs hollow sphere(HS)materials:SiO2 microspheres were used as templates,the surface of which was modified by APTES to provide-NH2 "anchoring sites".Then the crystal seeds were grafted via layer-by-layer self-assembly growth of COFs.Transferring crystal seeds in solvothermal secondary growth system,SiO2@COFs core-shell structure was prepared.After etching SiO2 cores by HF aqueous,COFs HS were finally obtained.In the experiment,according to the Schiff-base reactions between Tp and Pa or BD,TpPa COFs HS with 1.8 nm pore size or TpBD COFs HS with 2.3 nm pore size was successfully synthesized.Moreover,using SiO2 microspheres with different diameters as template,a series of TpPa COFs HS with diameters of 400,500 and 600 nm were further synthesized.(2)Controlled release property of TpPa COFs HS for 5-fluorouracil(5-Fu):5-Fu was loaded into TpPa COFs HS material via simple impregnation method.Thermogravimetric curves showed that the drug-loading content of TpPa COFs HS reached 38.5%used hexane as impregnating solution.The release rate was tested in simulated body fluid and TpPa COFs HS materials showed a good sustained release effect.The pH response test demonstrated that 5-Fu@TpPa COFs possessed a faster drug release rate with the decrease of pH.In order to give full play to the interaction between TpPa COFs and drug molecules thereby enhancing the sustained release effect,5-Fu was loaded into amino-modified SiO2 hollow mesoporous spheres by impregnation method.After 30 layer-by-layer growth cycles of TpPa COFs,5-Fu was encapsulated in TpPa COFs.Release rate test in simulated body fluid showed that 5-Fu@SiO2@TpPa COFs had a better sustained release and pH-responsive effects.(3)Catalytic performance of(Pd/C)@TpPa COFs on Suzuki coupling reaction:Using CTAB as soft template,the core-shell structured(Pd/C)@SiO2 was prepared,and the surface was modified by APTES to provide the-NH2 "anchor reaction sites"for the growth of COFs.After solvothermal reaction and HF etching,(Pd/C)@TpPa COFs was synthesized.EDS results showed that the palladium content(Pd/C)@TpPa COFs catalyst could be adjusted in the range of 3%-20%by simply changing the additive amount of Pd(NO3)2 in the synthesis process.At ambient temperature and in aqueous solution system,Suzuki coupling reaction results showed that under the catalysis of(Pd/C)@TpPa COFs,the conversion ratios of reaction between bromobenzene and o-,m-and p-methylphenylboronic acid were 14.5%,57.3%and 33.5%respectively.However,in the same system,the conversion ratios were 10.4%,88.0%and 82.0%respectively under the catalysis of Pd/C.Due to the existence of TpPa COFs shells,(Pd/C)@TpPa COFs catalyzer possessed an excellent shape selectivity.In the stability test,the catalytic conversion of bromobenzene to phenylboric acid maintaind between 60%and 70%for 5 consecutive reactions,indicating a favorable stability of(Pd/C)@TpPa COFs.In summary,to give full play of the well-defined pore structure of COFs materials,a series of crystalline COFs hollow spheres with different particle diameters and pore sizes have been successfully synthetized by template method,and their properties in drug controlled release and Suzuki catalysis have been preliminarily studied.The enhancement of drug controlled release performance and the improvement of catalytic selectivity and stability of Suzuki reaction have been realized.This work provides a new idea for strengthening the application properties of COFs materials.