Design,preparation And Properties Research Of 2d[4+3]schiff Base Covalent Organic Frameworks

Covalent Organic Frameworks(COFs)are crystalline organic porous materials formed by small organic molecules through dynamic chemical bonds,usually composed of C,H,O,N,B,Si and other light atoms,with features of low density,high specific surface area and stable chemical properties.The channel environment of COFs,symmetry and the connection of construction units are all designable,and the physical and chemical properties of COFs can be precisely regulated.The nanoporous channel of COFs makes it an ideal material for gas storage and separation,and its high specific surface area and certain interface environment make it have potential applications in catalysis and sensing.At the same time,the regularities of the structure and the modulability of the organic building blocks allow COFs to be used in fields such as optoelectronics and batteries.Excellent physical and chemical properties and wide application prospects have made COFs become a current research hotspot.Compared to(Metal Organic Frameworks)MOFs,COFs has similar structure and network chemical design principles,but there is a big gap between the diversity of COFs topological structure and MOFs',the restriction of topological structure type is still the bottleneck to limit the development of COFs' diversity.In addition,functional application of COFs is also one of the important research directions in this field,and functional modification of COFs is still the premise of COFs'application.At present,the connection reaction of COFs modification and the selection range of modifiable COFs construction units are still relatively limited,so it becomes more and more important to propose a COFs construction method that can realize the preparation of COFs with modifiable sites by using common COFs construction units.In this paper,a new[4+3]two-dimensional COFs construction strategy was proposed,and two new COFs topological types,tth and bex,were proposed.The construction strategy of unsaturated connection was adopted,and the aldehyde-rich tth ETTA-TFPT COF with specific surface area up to 750 m2 g-1 was prepared by taking advantage of the steric hindrance step between construction units.The structure simulation and refinement of ETTA-TFPT COF were carried out by software,and the topological configuration of ETTA-TFPT COF was proved by combining with the pore size calculation results of isothermal nitrogen adsorption non-localized density functional theory(NLDFT).Combined with the lamellar structure of COFs in the TEM image,the two-dimensional property of COFs was strongly proved,and the existence of the linker aldehyde group was verified by infrared and solid nuclear magnetic resonance.Based on the successful synthesis of 4A-tth ETTA-TFPT COF,ETTB-TAPT COF and ETBC-TAPT COF were designed and synthesized.The effects of the type and size of building units on COFs topology and functional group retention were studied,and the versatility of the[4+3]construction strategy was verified at the same time.The study found that the aldehyde groups were more stable during the COFs constructed by the[4+3]strategy.Therefore,in the[4+3]construction strategy,when the aldehyde-based building block reaction functionality has 3 reaction sites,the COFs may be likely in the tth topology configuration.In other words,when the aldehyde-based building unit has 4 functionalities,the COFs are more inclined to bex topological configuration.In this paper,ETBC-TAPT COF with excellent thermal stability has two kinds of pore types,mesoporous and microporous,and its specific surface area is as high as 1987 m2 g-1.The adsorption performance of COFs in sulfur hexafluoride gas adsorption was studied for the first time.The sulphur hexafluoride adsorption performance of ETTB-TAPT COF(molecular dynamics pore size 1.34 nm)and ETBC-TAPT COF(molecular dynamics pore size 1.1 nm)was investigated.Among two kinds of COFs,ETBC-TAPT COF with higher specific surface area and optimum sulfur hexafluoride adsorption pore size(1.1 nm)has a larger sulphur hexafluoride gas adsorption capacity of 4.4 mmol g'1,high sulphur hexafluoride selective adsorption,good cyclic adsorption and the medium heat of adsorption,which mean that ETBC-TAPT COF has great application prospects in the field of sulfur hexafluoride adsorption.To sum up,this paper proposes a new[4+3]COFs construction strategy,which realizes the one-step preparation of COFs with reserved reactive groups using general construction units.And proposes two new COFs topological structures,tth and bex,which enriches the development of COFs topological structure diversity and provides ideas for preparation of COFs with reserving groups in the skeleton.