| Covalent organic framework(COF),as low density crystalline porous materials with periodically ordered skeletons,is generally composed of organic molecules connected by covalent bonds.Benefiting from the advantages of accurately predictable structure,easy design,low skeletal density and high stability,COF materials have shown great potential in the fields of heterogeneous catalysis,optics,electrochemistry,biomedicine,gas storage and separation,especially for hydrogen storage.According to the connectivity and symmetry of monomers,COF is mainly divided into two types with two-dimensional(2D)and three-dimensional(3D)networks.2D COF with one-dimensional channels is stacked by?-?interactions and3D COF commonly possesses interconnected porous structures.So far,the preparations of 2D COF have been mature due to the diverse building units.But the applications and developments of 2D COF are limited due to the characteristics of single channel and few available active sites.Compared with 2D COF,3D COF has attracted extensive attention from scientists due to their irreplaceable advantages such as cross-linked channels,high specific surface area and rich active sites.Therefore,it is exceedingly significative to construct 3D COF.However,the developments of 3D architectures remain a great challenge:(1)available building blocks and reversible bound types are limited,(2)the formation of high-quality single crystals is difficult due to relatively low reversibility,and(3)the characterization technique analysis is restricted and hard to determine the structure.As the global population continues to grow,so does the demand and consumption of energy.Fossil fuels,as a non-renewable energy source,provide more than 80%of the global energy consumption,but their contents are very limited.In addition,the burning of fossil fuels has also brought serious disasters to organisms and the environment,such as greenhouse effect,acid rain,haze,respiratory problems and so on.Hydrogen,as a pollution-free,renewable and efficient clean energy,will inevitably become the mainstream energy to replace fossil fuels in the future.However,due to its extremely low volume energy density(0.01 MJ/L),it becomes a major research focus and challenge in the field of exploring suitable and efficient hydrogen storage methods.Traditional hydrogen storage methods,such as high pressure,low temperature and conversion into liquid hydrogen-rich molecules,require strict storage conditions and expensive costs,which are difficult to meet long-term practical applications.Solid-state nanomaterials for hydrogen storage are generally considered to be safer and more practical method for hydrogen storage due to their stable energy state,low operational barriers and on-demand release characteristics.According to literature research,the main influencing factors of hydrogen storage in solid nanomaterials include“respiration effect”,surface area,pore volume and active site.COF,especially 3D COF,as a kind of solid nano hydrogen storage material,is expected to become an excellent hydrogen storage material due to its advantages of distinct structure,modifiable skeleton,adjustable aperture,low skeleton density,high specific surface area and good stability.In view of the above considerations,three new types of COF materials with dia,hea and stp topologies were designed,synthesized by selecting appropriate structural unit combinations(Td+C2)?(D3h+Td)?(D3h+C2)under the guidance of reticular chemistry based on topological structure,and their effects in hydrogen storage were studied from following aspects:solving the“respiration effect”of COF with a certain penetration,designing a new topology with multiple active sites,improving specific surface area and comparing topologies.The research results are mainly divided into the following three parts:(1)According to literature research,we found that the structures of 3D dia COF are easy to be interpenetrated and synthesized,but they often have the problem of low gas storage due to“breathing effect”.In order to solve this problem,we plan to introduce an asymmetric rigid side chain to the skeleton of the material in a certain direction in this chapter,and it would replace the effect of solvent,which would support the framework to prevent the contraction of the material frame structure while keeping the original structure unchanged,and thus inhibiting the“breathing effect”.In particular,we chose stereotetra-linked 1,3,5,7-tetrakis(4-formylphenyl)adamantane monomer to combine with 2-connected 4,4'-diaminobiphenyl and 3,8-diamino-6-phenylphenanthridine,respectively.Then,we successfully synthesized two 3D dia COF with two penetrating structure,termed JUC-594 and JUC-595,and its hydrogen storage performance was also studied.JUC-595 maintains the original rigid structure due to the introduction of asymmetric rigid side chains compared with JUC-594,and solves the“breathing effect”,which effectively improves the specific surface area of the material and its hydrogen storage capacity.(2)In our first work,although the“breathing effect”of COF with dia topology has been solved and its hydrogen storage capacity is also improved,but such adsorption amount is not ideal.The structure determines the properties,it may be the reason for the low specific surface area of COF with dia topology themselves in the absence of specific adsorption sites.Therefore,to further improve the hydrogen storage capacity of COF,we try to design new topologies with active sites.Two isomorphic non-interpenetrated hea net 3D COF(JUC-596 and JUC-597)were successfully synthesized by the schiff base reaction between 6-linked aldehyde monomer and its derivatives based on tridenes and tetra-[(2-fluoro-4-aminophenyl)phenyl]methane.Interestingly,this is the first 3D COF with non-interpenetrated hea reticular structure.In addition,the hydrogen storage properties of these two 3D COF materials were also studied due to the presence of triptycene functional units.The results show that JUC-596 exhibits the expected more excellent hydrogen adsorption performance at 1 bar,which is higher than that of almost all porous materials reported so far.In view of this,JUC-596 was also tested for high-pressure hydrogen adsorption at room temperature,and its hydrogen excess adsorption and total adsorption at 80 bar were comparable to MOF materials with bare metal sites.In addition,we carried on the theoretical calculation by grand canonical Monte Carlo(GCMC)simulations based on the RASPA package,and clearly explained the reasons for the large difference in the hydrogen adsorption capacity of the two isomorphic 3D COF and the effect of fluorine on the crystallinity.(3)From literature reports and previous work,we concluded that both high specific surface area and functionalization of triptycene and fluorine atoms are beneficial for hydrogen adsorption,but it is an extreme challenge to design materials that simultaneously achieve high specific surface area and multiple active sites.According to literature reports,the COF with stp topology has the largest specific surface area among the 3D schiff base COF reported so far.At the same time,we explore to solve the crystallinity problem left in the second work.Therefore,we try to replace the stereo four-connected building unit with the planar four-connected building unit.Specifically,we designed and prepared a stp COF material(JUC-598)functionalized with triptycene and fluorine atoms by using 6-linked 2,3,6,7,14,15-hexa(3'-fluorine-'-formylphenyl)triptycene and 4-[3,6,8-tris(4-aminophenyl)pyren-1-yl]aniline,and studied its hydrogen adsorption performance.The results show that the structure of JUC-598 is stp topology,as we expected,and can adsorb a certain amount of hydrogen,but it is lower than expected.To explore the reasons,we conducted a detailed hydrogen storage mechanism analysis,which provides a very important and meaningful idea for our subsequent design of COF materials for improving hydrogen storage. |
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