Design,Synthesis And Application Of Three-dimensional Covalent Organic Frameworks With Stp Topology

As porous materials continue to evolve and iterate,an increasing number of such materials are being explored and discovered.Inorganic porous materials were the first to be discovered and applied in production and daily life,followed by inorganic-organic hybrid porous materials that have attracted widespread research interest.Currently,pure organic porous materials,particularly covalent organic frameworks（COFs）,has attracted great interest among researchers.COFs are composed of organic structural units linked by covalent bonds and are a type of pure organic porous material.They possess several unique advantages that other porous materials lack,such as crystalline structures,regular and controllable pore structures,highly tunable functionalized frameworks,high specific surface areas,and excellent structural stability.From the perspective of the dimensions of crystal growth,COFs can be mainly classified into two-dimensional and three-dimensional COFs.Unlike the numerous two-dimensional COFs,three-dimensional COFs still face significant challenges in preparation conditions and structural analysis,which is the primary reason why the research depth and breadth of three-dimensional COFs are far below those of similar materials.However,three-dimensional COFs possess unique advantages such as higher specific surface area,more open active sites,and interconnected pores,which make the materials potential in various fields.Energy scarcity and environmental pollution remain major obstacles to global economic development and human progress.Along with efforts to conserve energy and improve efficiency,developing and harnessing new sources of clean energy has become a pressing concern for nations worldwide.In recent years,China has implemented the strategy of"Carbon Peak and Carbon Neutrality"to address energy and environmental issues.Among the many emerging materials and scientific technologies,the photocatalytic CO₂ reduction was considered to be one of the most promising ways to achieve this target.In particular,the application of photocatalytic CO₂ reduction technology addresses the climate change issue caused by CO₂ emissions and helps to further solve the challenges of energy scarcity and clean energy development.To address these concerns,we have developed and synthesized a range of innovative three-dimensional covalent organic frameworks（3D COFs）.Employing established techniques,we have conducted a thorough characterization of their structures and properties,with a focus on exploring the potential applications of 3D functionalized COFs in the field of photocatalytic CO₂ reduction.The specific scope of our research is detailed below:（1）Taking into consideration the lack of reported topological structures and the issue of monofunctional organic building units in current 3D covalent organic frameworks（COFs）,we have synthesized,a trisubstituted triptycene-based organic building unit firstly.Due to its novel D3h symmetry,we were able to further synthesize the first example of 3D COFs,JUC-564,with the stp topology under solvothermal conditions.Through experimental powder X-ray diffraction combined with theoretical modeling,we have confirmed its stp topology.By utilizing nitrogen sorption-desorption,SEM,and TEM characterization techniques,we have demonstrated that JUC-564 possesses mesoporous channels of 4.3 nm,a high specific surface area of 3383 m² g^-1,and a low skeletal density as low as 0.108 g cm^-3.Additionally,the adsorption experiment of the biomacromolecule myoglobin further confirmed the mesoporous channels of JUC-564.The work of this section offers a preliminary theoretical research basis for the development of 3D COFs in the direction of macromolecular loading transport（2）Based on the unique advantages of JUC-564 in terms of specific surface area,skeletal density,and mesoporous pore size,we developed post-synthetic modification strategies to obtain respectively amide-link JUC-564-O under oxidative conditions and quinoline-link JUC-564-P under cycloaddition conditions.Experimental results using PXRD,nitrogen adsorption-desorption,IR spectroscopy,SEM,and TEM showed that the crystallinity,specific surface area,pore size,and morphology of the two modified COFs did not change significantly,indicating the feasibility and operability of the post-synthetic modification strategy for JUC-564 with imine linkages.Furthermore,stability tests using organic solvents and strong acid-base aqueous solutions showed that the crystallinity of the two modified COFs remained unchanged for at least 5 days,demonstrating that the chemical stability and structural rigidity of JUC-564-O and JUC-564-P were greatly improved,effectively solving the problem of poor stability of 3D imine-linked COFs.The high stability of mesoporous 3D COFs materials is conducive to further exploring their potential value in unknown fields and may even be applicable in practical production and life.（3）Based on the stp topology,we selected porphyrin and metalloporphyrin with photoelectric activity,and synthesized a series of non-interpenetrated stp topology JUC-640-M（M=H,Co,Ni）through in situ functionalization strategy and assembly with the D_3h symmetry of the triptycene.With the unique advantage of stp topology,the JUC-564-M framework structure has abundant porphyrin active sites(0.845 mmol g^-1).To further explore the photoelectric properties and catalytic performance of 3D COFs as photocatalysts,we prepared a series of JUC-640-M-based photocatalysts for the study of photocatalytic CO₂ reduction process.From these experimental results,compared with the other two porphyrin-based COFs catalysts,JUC-640-Co had the highest CO production rate of up to 15.1 mmol g^-1 h^-1,which is currently the highest yield value among known COFs-based photocatalysts.Moreover,JUC-640-Co also exhibited a high selectivity of 94.4%and cycling stability.