Preparation And Separation Properties Of Porous Covalent Organic Framework Blocks Based On Pickering Emulsion Template Method

Covalent organic frameworks(COFs)are the type of orderly crystalline porous organic materials constructed by organic building units consisting of light elements(C,O,B,and N)via the reversible polymerization reactions.As an emerging class of porous crystalline polymer,COFs materials hold the advantages of organic porous materials,such as low density,good stability,high specific surface area,easy functionalization and so on.More importantly,COFs materials possess unique properties,for example,the controllable porous size and surface chemistry adjusted from a thoughtful selection of building units prior to synthesis,the reversible covalent bonds between monomers,the various synthesized methods,and the good crystallinity.Due to their special structures and unusual physical-chemical performances as the above mentioned,COFs have recently attracted considerable attention as promising many applications in adsorption separation,chemical sensing,heterogeneous catalysis and other fields.To date,many methods,including solvothermal method,ionothermal method,microwave method,room temperature method and solvent stripping method have been developed to prepare COFs materials,and the resulting products are mainly in shape of micro-nanoparticles.As we all known,the assembly and integration of nanomaterials are the basis of practical applications of nanomaterials and meanwhile the individual properties of single nanoparticle can be integrated and enlarged to generate new functionality.But how to assemble these nanoscale building blocks into advanced hierarchical structures is still a big challenge.To our best knowledge,the general and important strategy is the template-assisted assembly.As far as templated are concerned,emulsion systems as an important type of liquid template have attracted much attention because of its simplicity,economy,easy control and scale-up preparation.As a thermodynamically unstable system,however,the emulsions can serve as the successful templates determined by the performances of the liquid-liquid interfacial film involved the structural characteristics,rheological properties and evolution process,which are also decisive factors for the formation and fine regulation of porous structures of the resulting materials.Our group has always kept on fabrication the practical value porous materials with macroscopic 3D architectures.Our studies were mainly focused on the preparation,the structured control,and functionization of the novel porous materials via emulsion templated strategy in these years.Furthermore,effects of properties of interfacial film on the stability of the starting emulsions and porous structures of the resulting materials have investigated by tuning the chemistry of amphiphilic molecules.COFs,represented an emerging class of materials,have interestingly studied in pre-design of monomers,stability of COFs particles/films,and performances of COFs.But the efficient preparation and controllable structures of COFs materials with macroscopic 3D architectures is the key to implement potential applications of COFs.Unfortunately,there are few reports to macroscopic 3D COFs materials.As part of our ongoing research in the fabrication of novel porous macroscopic 3D materials via emulsion template,it is of great significance and feasibility to successfully prepared macroscopic 3D COFs materials via emulsion template-assisted assembly of COFs particles.Undoubtedly,to do this have to face a few difficulties,such as formation of stable emulsion systems,control of the interfacial properties,adjustment of mechanical performance of the resulting materials,the integration of COFs micro-nanoparticle properties and so on.Based on the research status of COFs materials and the studies of porous materials prepared by emulsion template method in our group,this dissertation is devoted to developing the Pickering emulsion template method for COFs with macroscopic 3D architectures.We firstly designed and synthesized COF solid particles with amphiphilic properties as solid particle stabilizers,which are able to adsorb on the oil-water interface,producing a series of stable Pickering emulsion systems.A series of COF monolithics(COF-Ms)materials were then successfully fabricated without destroying the porous structures due to effectively restraining the capillary stress with the help of freeze-drying.It was demonstrated that the COF-Ms materials not only have good porosity and thermal stability,but also possess excellent elasticity and cutability.More interestingly,the as prepared COF-Ms materials can serve as disk-like separation column,realizing the efficient separation of p-nitrobenzene isomers.Specifically,the major work was done in the following two aspects:In the first part of the thesis,COF particles with amphiphilic structure were synthesized by the "Schiff base" reaction connected via imine bonds,and the COF particle combined with cholesterol(Chol-OH)small molecular compound presiously reported in our group were then used as composite stabilizer,which is denoted as COF/Chol-OH.The as prepared the composite stabilizer is able to emulsify the mixture of an aqueous solution containing PVA(as the aqueous phase)and mesitylene(as the oil phase),producing a series of O/W Pickering emulsion systems through a one-step emulsification method.The results showed that the stability and morphology of the resulting Pickering emulsion systems were adjusted by varying the water phase contents,COF particle contents and PVA contents.On the basis of the starting Pickering emulsions as templates,a series of COF-Ms materials were fabricated with the help of freeze-drying,the micro-channel structure and porosity of the as prepared COF-Ms materials were then tuned by a change in formulation of emulsions.The results showed that Chol-OH can effectively improve the amphiphilicity of COF particles,which is confirmed by increasing the water contact angle from 59.5° of bare COF particles to 74.5°of COF/Chol-OH paritcles.The better amphiphilic performance of the composite stabilizers property make them easily adsorbed on the oil water interface,forming rigid interface film,which effectively inhibits the coalescence and rupture of emulsion droplets.Using as prepared Pickering emulsion as a template,the macroscopic 3D COF-Ms with hierarchical porous structure can be successfully prepared via the assembly of COF particles at oil-water interface,followed by freeze drying.The porous structure,mechanical property,and densityof as prepared monolithic materials can be adjusted in a large range by a change in the formulation of the starting emulsions,such as water content,PVA content and COF particle content.Taking the COF-M4 material with 40%water phase as an example,the COF-M4 material showed the good thermal stability and elasticity,wherein COF-M4 can be arbitrarily cut.In the second part of the thesis,the separation device was firstly set up,the separation capacities of the COF-M4 material for nitrotoluene isomer,chloronitrobenzene isomer,xylene isomer and dinitrotoluene isomer were then studied by using the disc-like COF-M4(10 mm i.d.×1-5 mm height)as the separation column.We investigated the effects of mobile phase composition,flow rate and column height on separation efficiency.The experimental results showed that the COF-M4 separation column exhibit the excellent separation efficiency of nitrotoluene isomer and chloronitrobenzene isomer,and the recovery rate was about 85%.Inversely,separationof xylene isomer and dinitrotoluene isomer cannot be realized by as prepared COF-M4 column.The experiment showed that after three cycles of recycling,the separation and recovery efficiencies were almost not attenuated,which testified that the channel structure of the COF-M4 material is very stable and no any destruction after five usage.Obviously,the COF-M4 material has a potential application as separation material.