Construction Of Mesoporous Cages In MOF Using A Small Pyrazole Carboxylate Linker

Metal-organic framework?MOF?is an emerging class of porous materials which have high surface area,structural and functional tunability.They have shown great potential applications in gas storage,separations,catalysis and biomolecule inclusion.Scientists have been pursuing the synthesis of MOF materials with large open pores,thermal and chemical stability,so as to introduce different guest into the pores and expand the applications in various fields.However,the main strategy to mesoporous MOF?>2 nm?is extending organic linkers or using large ones.This method involves the complicated and tedious organic synthesis process,which is time-consuming and costly.Therefore,it is of great significance to develop a simple and feasible method to construct mesoporous MOF for their further development and applications.This thesis mainly focus on the synthesis of stable MOF using small organic linker.A small di-topic linker,1H-pyrazole-4-carboxylic acid?H₂PyC,0.4 nm in length?was selected to construct the mesoporous MOF.The linker has two different terminals,one is carboxyl terminal and the other one is nitrogen terminal.The two are coordinated with different metal salts to form two 3D frameworks,MOF-818 and MOF-919.The as-synthesized MOF were activated by solvent exchange or supercritical CO₂ drying,then N₂ absorption experiments were tested and given the BET surface area of MOF-818 and MOF-919-Sc were 2050 m²/g and 2740 m²/g respectively.At the same time,the characterization and stability of these two mesoporous MOF?MOF-818 and MOF-919-M,M=Fe,Sc,Al?were tested and the results have showed good thermal and chemical stability?base and acid stability?of MOF-818.Based on the small angle X-ray scattering?SAXS?,synchrotron PXRD and 3D electron diffraction pattern,the structure of MOF-818 and MOF-919-Sc were studied by Rietveld refinement.Three mesoporous cages of unprecedented polyhedra are identified in these MOF,wuh cage in MOF-818,yys and liu cages in MOF-919,with the diameter of 3.8,4.9 and 6.0 nm,respectively.In order to check the ultra-high porosity and mesoporous nature of the synthesized MOF,inclusion of biomolecules,vitamin-B₁₂(VB₁₂)and insulin,are researched here.The largest size of VB₁₂ and insulin are about 2.7 and 3.4 nm,respectively,which match well with the pore size and aperture of MOF-818 and MOF-919.Inclusion study of VB₁₂ or insulin showed these biomolecules can enter the pores of MOF-818 or MOF-919-Sc easily,indicating the porosity and mesoporous nature of MOF-818 and MOF-919.Finally,it is necessary to extract the cage structure in 3D networks to fairly compared with supramolecular polyhedra.For example,in the crystal structure of MOF-818 and MOF-919,the di-topic H₂PyC linker functions as the edge in the structure,while two types of metal-containing SBUs function as the vertices.After that the systematic analysis of supramolecular cages,3D extended cages in MOF and the synthesized cages are carried out in the thesis,revealing the guiding principle for the synthesis of large cages.They are?1?The size of polyhedra increases as the number of vertices grows without altering the connectivity of vertices;?2?When the number of vertices is relatively the same,polyhedra with less connectivity usually exhibit a larger size.All three cages in this thesis are constructed by three-connecting?3-c?vertices and two-connecting?2-c?vertices.This[2-c,3-c]connectivity represents the least connectivity required for the construction of cage and the most effective way for cage size expansion.The largest cage liu,exhibits a cage size to linker size ratio of 15,outstanding in supramolecules and MOF.