Design Of Hierarchically Porous Metal-organic Frameworks And Its Derived Semiconductor Applications

Metal-organic frameworks?MOFs?are crystalline porous materials constructed through the formation of coordination bonds between metal ions or metal clusters as nodes and organic ligands as spokes.With advantages of large specific surface area,high porosity and designability,MOFs has been widely used in many fields,especially in catalysis.Since Yaghi first synthesized MOF using carboxylic acid as ligands in the 1990s,the development of MOFs has been rapid,with over 20,000 MOFs being synthesized to date.However,most of the MOFs have only microporous structures with small pore size?<2nm?,which undoubtedly limits the transmission and diffusion of macromolecules.Macromolecules are unable to access to more active sites,so the catalytic process requires more time and energy,which is not consistent with the concept of green and sustainable development.As a potential catalyst,the introduction of mesoporous and macroporous structures into the structure of MOFs is of great significance to the improvement of their catalytic performance and catalytic efficiency.In this paper,MIL-125?Ti?/1a with macroporous-mesoporous structure was synthesized with distilled ionic liquid 1a as solvent in one-step,high efficiency and green without adding template agent.The MOF shows a sponge-like architecture with a significant macropores with the size ranging from 50 to 300 nm.The mean size of mesopore is 7.6 nm and macropoe is 223nm.Then,the material was characterized and compared with MIL-125?Ti?,and the results showed that the two materials were consistent in chemical composition,element and valence state,bonding information and thermal stability,in addition to crystalline structures.The influence of the type and concentration of distillable ionic liquid and reaction time on the morphology of MIL-125?Ti?/1a was further investigated.The formation path and mechanism of hierarchically pore structure were explored through the treatment of MIL-125?Ti?by alkali and distillable ionic liquid 1a.The results showed that,even if the amine compounds of the distillable ionic liquid were different,they could be used for the construction of hierarchically porous MOF,and with the decrease of the amount of distillable ionic liquid,the morphology of MOF could be changed from sponge-like architecture to sphere.In this system,the distilled ionic liquid has three functions:mechanical disturbance,organic base and pore expanding agents.This kind of ionic liquid releases CO₂ and organic amine gas at high temperature.Organic amine accelerates the deprotonation of terephthalic acid and promotes the construction of nanosized framework of Ti-DBC.At the same time,organic amine as an organic base,coordinate to the Lewis acid Ti⁴⁺and destroyed the Ti-DBC structure,which caused the transformation and reconstruction,and finally formed hierarchically porous MOF.The method is applicable to the formation of Ui O-66?Zr?/1a with macroporous-mesopore structure,and the average mesopore aperture of which is 10.1 nm and macropore aperture is 360 nm.C-Ti O₂/1a_?600?,a carbon-doped titanium dioxide with MIL-125?Ti?/1a as the precursor,inherits the macroporous-mesoporous structure of the MOF,with an average mesoporous pore diameter of 12.4 nm and an average macroporous pore diameter of 182 nm.The hierarchically porous semiconductor showed excellent catalytic performance in the synthesis of imine and2-?methyl?-1H-benzo[d]imidazole.Finally,a preliminary study on the construction of composite MOF by distillable ionic liquid 1a was carried out,and it was found that a certain proportion of ZIF-9 precursors and MIL-125 precursors could generate composite MOF in the reaction system.The morphology of the material is related to the feeding ratio and reaction time.After optimization,Co_0.5-Ti_0.5-MOF shows a uniform hexagonal plate shape.On this basis,the method was further extended and it was found that the precursor of ZIF-9 and Ui O-66?Zr?could also form Co_0.4-Zr_0.6-MOF with hexagonal shape.