Tailoring Micro/Nanostructures Of Imine-based Two-dimensional Covalent Organic Frameworks

Covalent Organic Frameworks(COFs)are new organic crystalline nano-porous materials.Because of COFs’stable skeleton structure,adjustable pore structure,easily functionalized pore wall,and high specific surface area.COFs have potential applications in separation,catalysis,sensing,etc.However,most of the reported COFs are amorphous spherical-like particles with irregular morphology,and the micro nanostructure is difficult to be accurately customized,which greatly limits its application.Therefore,in this paper,the effects of monomer types,reaction nucleation diffusion kinetics and post-reaction conversion on the micro-nanostructure of imine-based two-dimensional(2D)COFs were systematically studied in the presence of monofunctional regulators.Flower-shaped particles with periodic orientation structure,micro nanoparticles with core-shell and hollow structure were prepared,and their applications in the fields of sensing,anti-counterfeiting,and catalysis were explored.The research of this paper has important scientific significance and practical value for promoting the application of COFs.The main contents include:(1)Given the unclear influence mechanism of molecular interaction on product assembly in COFs reaction system.The influence of nine monomer combinations with different molecular distortion angles on theπ-πstacking force between 2D COFs layers was investigated,and the influence of the force on the micro-nano morphology of 2D COFs aggregates was systematically studied."Flower"like particles were prepared,which were formed by periodic arrangement of highly oriented COF nanosheet units.The product has high crystallinity and a specific surface area of 1453 m~2/g.It has birefringence in the visible near-infrared band.It can be used as a micro polarization sensor for explosive molecular detection and new information material for optical anti-counterfeiting.(2)In view of the challenge of precise regulation of crystallization nucleation rate and diffusion rate of polycondensation products in COFs reaction system,single functional monomers(aniline and benzaldehyde)were used to regulate the reaction nucleation rate,and COFs particle seeds heterogeneous nucleation were introduced to regulate the nucleation diffusion rate.By matching the nucleation and diffusion rate of reaction,the efficient and accurate customization of core-shell micro-nanoparticles is realized.The product has high crystallinity,a specific surface area of 834 m~2/g,core and shell pore sizes are 2.5 nm and 1.7 nm,respectively.The product can be used as a carrier to support nano palladium catalyst to realize selective catalysis of substrates with molecular sizes and enhance the recycling stability of the catalyst.(3)Based on the above work,a post-conversion preparation strategy of COFs hollow structure is proposed.In the equilibrium reaction system containing aniline and benzaldehyde regulators,amorphous COFs particles with low crystallinity can be transformed into hollow COFs particles with a high crystalline state in situ.The specific surface area of the product is 543 m~2/g,and the pore size is 1.7 nm.Hollow COF particles with different particle sizes(120～700 nm)and wall thickness(25～125 nm)can be customized by controlling the particle size of amorphous COFs particles and adjusting the equivalent of aniline and benzaldehyde.