Controllable Synthesis And Property Studies Of High Pyridine Nitrogen-doped Mesoporous Carbon Materials

Mesoporous carbon materials have the advantages of high chemical and thermal stability,easy processing and modification of carbon-based materials,as well as high specific surface area,large pore volume,special pore structure,adjustable mesopore size and other characteristic properties of mesoporous materials,making them widely used in the fields of catalysis,adsorption,gas-sensitive and energy conversion and storage,which have attracted widespread attention.The introduction of nitrogen atoms into the carbon matrix is an effective way to improve the intrinsic physicochemical properties of the carbon skeleton,which can greatly improve the surface wettability,electron donor properties,electrical conductivity and reactivity of carbon materials.However,so far,the synthesis and application of nitrogen-doped mesoporous carbon materials still suffer from the following problems:（1）The in-situ synthesis of nitrogen-doped mesoporous carbon materials can use few types of monomer molecules,and the reaction system is limited.The self-assembly of surfactants to form micelles and self-assembly with monomer molecules often requires special solvent systems,which limits the development of nitrogen-doped mesoporous carbon materials to a certain extent.（2）The nitrogen content of existing nitrogen-doped mesoporous carbon is low,and the ratio of species（especially the content of highly reactive pyridine nitrogen species）is low and difficult to regulate effectively,so it is difficult to investigate the influence of its composition and content on the catalytic,energy storage and adsorption properties;（3）The existing nitrogen-doped mesoporous carbons often suffer from small pore size,uncontrollable structure,and uneven distribution of nitrogen atoms（especially for the material obtained by post-processing method）and other issues.Therefore,the synthesis and fine structure control of high nitrogen-doped mesoporous carbon materials still face great challenges,which greatly limits the development and application of nitrogen-doped mesoporous carbon materials.It is of great significance to explore new monomer molecules and reaction systems to realize the controllable synthesis of nitrogen content,nitrogen species,and their fine structures in nitrogen-doped mesoporous carbon materials.This thesis aims to explore and optimize the synthesis of nitrogen-doped mesoporous carbon materials,design and synthesize various nitrogen-doped mesoporous carbon materials with tunable structure,and realize the controlled synthesis of the nitrogen content,nitrogen species,and fine structure of the materials,and systematically study the structure-activity relationship between material structure and nitrogen species and material properties.1.We developed 2,6-diaminopyridine monomer,and achieved fine control of the hollow structure of Schiff base polymer colloidal spheres and nitrogen-doped carbon microspheres through gradient growth and confinement polymerization strategies.The strategy is based on the Schiff base reaction between 2,6-diaminopyridine（DAP）and terephthalaldehyde（TPA）,F127 and sodium dodecylbenzenesulfonate（SDBS）dual surfactants as structure directing agent.By changing the mass ratio of SDBS to F127,the average size of the polymer colloid spheres can be linearly adjusted from 180 to1720 nm.In this process,the synthesis of polymer colloid spheres from hollow to multichamber surrounded hollow,and then adjusted to a multichamber structure was also realized.These polymer colloidal spheres have good thermal stability and are carbonized into corresponding nitrogen-doped porous carbon nanospheres（SBCs）with little change in their internal structures.Since SBCs have different internal structures and similar specific surface areas as well as nitrogen contents,which provides the possibility to investigate the relationship between internal structure and performance.As a supercapacitor electrode material,SBCs exhibit excellent capacitive performance,which is attributed to its fine internal structure.2.we propose a simple multi-stage self-assembly method for the synthesis of high pyridine nitrogen-doped mesoporous carbon materials.The method uses DAP as the monomer molecule,block copolymer polystyrene-b-polyethylene oxide（PS-b-PEO）self-assembled to form micelles as a mesoporous template,and graphene oxide（GO）as substrates.The tunable mesopore size from 8 to 25 nm was achieved by varying the length of the PS block in PS-b-PEO.By changing the calcination temperature,the nitrogen content can be adjusted from 14.4%to 22.4%.In addition,the ratio of pyridine nitrogen was always kept above 45%during this process.The synthesis of zero-and one-dimensional composites was achieved by using different substrates.The obtained two-dimensional highly nitrogen-doped mesoporous carbon materials exhibit excellent area-normalized capacitance(90.6μF cm^-2)in the supercapacitor test,which is much higher than the theoretical electrochemical double-layer capacitance（EDLC）of activated carbon(15-25μF cm^-2),as well as some previously reported porous carbon materials.In addition,the material exhibits excellent cycling stability at high current densities.3.To expand the synthesis methods of 2D mesoporous materials,we further propose a multi-dimensional molecular self-assembly strategy,which is based on the self-polymerization reaction of DAP molecules with abundant amino groups,the self-assembly of PS-b-PEO to form spherical micelles as mesoporous templates,and perfluorotetradecanoic acid（PFCA）to form two-dimensional supramolecular assemblies as 2D substrates,and the co-assembly driven by hydrogen bonding to obtain two-dimensional mesoporous poly（diaminopyridine）polymers（MPDAPs）.The MPDAPs possesses tunable mesopore pore size（17-35 nm）and high nitrogen content（18.0 at%）,which exhibited high catalytic activity for the Knoevenagel condensation reaction.In addition,after the obtained composites were calcined in nitrogen,two-dimensional high nitrogen-doped mesoporous carbons（NMCs）materials were obtained.Due to their high specific surface area and basic nitrogen（pyridine and pyrrolic nitrogen）contents,NMCs exhibit a CO₂adsorption density as high as 11.7μmol cm^-2,which is much higher than that of most reported porous materials.In addition,the material exhibited a high CO₂/N₂adsorption selectivity（44:1）.4.We have synthesized multi-structured highly pyridine nitrogen-doped porous carbon nanospheres using droplet template strategy.The controllable morphology of the porous carbon spheres is mainly achieved by controlling the self-assembly process of PS-b-PEO to build spherical and vesicle-shaped supramolecular assemblies.The tunable pore size of the porous carbon spheres was mainly achieved by changing the polymerization degree of the PS block in PS-b-PEO.The obtained N-doped hollow carbon nanospheres（NHCS）and mesoporous carbon nanospheres（NMCS）both showed high nitrogen content.For NHCS,the tunable carbon skeleton structure was achieved by using different precursor molecules.For NMCS,the nitrogen content was tunable from 9.9%to 23.0%by changing the calcination temperature.Due to the high nitrogen content and large mesoporous pore size,NMCS is regarded as an ideal catalyst support for stable ultrasmall Pd nanoparticles（Pd/NMCS）.The catalyst exhibited high catalytic activity and selectivity as well as good cycling stability in the hydrogenation of phenol to cyclohexanone.By controlling the amount of Pd loading,the size of the loaded Pd nanoparticles as well as the valence state,and the mesopore size of the support,we found that pyridine nitrogen and pyrrole nitrogen in NMCS carriers have a facilitating effect on the catalytic hydrogenation of phenol.