Synthesis And Properties Research Of Covalent Organic Frameworks

Covalent Organic frameworks is a class of porous crystalline materials constructed by light elements,such as carbon,hydrogen,oxygen,nitrogen and linked by reversible covalent bonds.The synthesis method of COFs is based on dynamic covalent chemical(DCC),which leads to the reversible formation of covalent bonds through formed,broken,and reformed.Therefore,unlike conventional covalent bond formation,DCC is thermodynamically controlled and thus offers reversible reaction systems with "error checking" and "proof-reading" characteristics,leading to the formation of the most thermodynamically stable COF materials.COF materials possess the advantages of low density,high surface area,uniform pores,high chemical and thermodynamic stability and tuneable properties and functionality.With this virtue,COFs can act as a molecular platform for devising multi-functional materials with wide application prospects,such as gas storage and separation,catalysis,energy storage and conversion,and optoelectronics.In this thesis,we focus on the characteristic of COFs between structure and property,and design and synthesize a class of novel COFs materials.These COFs combine special properties are achieved by tuning structure of the frameworks,and its application in COZ adsorption,photocatalytie CO2 reduction and fluorescent sensing have been studied.The main contents are as followsChapter 1,the background and current situation of COFs are briefly summarizeed The design idea and strategies,the synthesis methods and difficulties,the characterizations and the applications of COFs are introduced in detail.Chapter 2,N-rich covalent organic frameworks with different pore size for high-pressure CO2 adsorption.Three high surface area,high nitrogen content COFs with different pore size linked by Schiff-base bond are design and synthesis using bottom-up strategy.These characteristics are not only in favour of enhancing CO2 uptake capacities,but also help to study the influence of CO2 uptake among several factors.COF-SDU1 gets large CO2 uptake capacity of 741 mg g-1 at 298K under 45 bar,which is much higher than other porous materials and exhibits superiority of COFs on CO2 adsorption.The result that BET surface area plays the crucial role in determining the high-pressure CO2 storage capacity and pore size 1s more influential than N-content are obtained through comparing every two COFs.In adsorption process,it shows that N-content determines capacities at low pressure(<lbar),BET surface area plays the decisive role under relative low pressure(<25bar)and large pore size helps enhance the capacity under relative high pressure(>25bar).Computer simulation of high-pressure CO2 adsorption 1s conduct,and the result agrees well with the experimental data.Accordingly,COFs with large specific surface area and large pore size were the primary principles for designing the novel structures for high pressure CO2 storage and utilization,such as reduction to methanol and conversion to cyclic carbonates.Chapter 3,a novel one-dimensional covalent organic framework.A novel one-dimensional covalent organic framework is firstly synthesized using a porphyrin building block and a non-liner building block.Unlike previously reported two-dimensional porphyrin-base COFs that each porphyrin connects four liner building blocks to form two-dimensional network structures,each porphyrin building block connects four non-liner building blocks to form a one-dimensional band-shape structure in this COFs.The N2 adsorption-desorption isotherm of COF-K belongs to typical Type-I,indicating a microporous nature and the pore size is 1.25nm.Small pore size is beneficial for low pressure CO2 adsorption and the CO2 adsorption capacity of COF-K reaches to 89 mg g’1,which is higher than two-dimensional porphyrin-based COFs.The structure of COF-K obtained from computer simulation is identical with the structure we designed and the corresponding PXRD pattern agree with the experimental data.Moreover,the pore size receive from simulation 1s also the same with experimental data,which demonstrates a rational simulated structure.This strategy gives a new direction to build one-dimensional COFs and expand greatly the scope of the emerging materials.Chapter 4,metal porphyrin based covalent organic frameworks for photocatalytic CO2 reduction.Three Co-,Ni-,Mn-porphyrin based covalent organic frameworks are designed and synthesized from one step method.Metal Co and Ni are uniform distribution on porphyrin centre in the form of divalent ions and the metal Mn distributes on the surface of COF-K in the form ofMnO2.The structure andmorphology of three metaled COFs are not affected by introducing metal,but the band gap and energy level will be changed.Three COFs exhibit wide visible light absorption reaching to 700nm,which indicate a good visible light utilization and no more photosensitizer need to be added.The conduct band of COF-K is-0.21 eV,which is incapable for CO2 reduction.However,the conduct band of three metaled COFs are reduced after introducing metal to COF-K and the new conduct bands are meet the needs of CO2 reduction.Chapter 5,Perylene derivative based three dimensional covalent organic frameworks.N,N’-bis(cyclohexyl)-1,7-bis(2,5-dihydroxybenzaldehyde)-perylene-3,4,9,10-bis(dicarboximide)as an aldehyde building block,tetrakis(4-aminophenyl)methane as an amino building block are designed and synthesized.Novel three-dimensional fluorescent COF-PDI is tried to construct using these two building blocks.However,in view of the difficulty of synthesizing of three-dimensional COFs materials is much greater than that of two-dimensional COFs materials,COF-PDI has not yet been obtained in limited attempts.