Construction Of MOFs Based On Multicomponent Strategy And Study Of Gas Adsorption And Separation Property

Metal–organic frameworks(MOFs)are one of crystalline materials which are constructed by inorganic metal ions and organic ligands.Since 1995,MOFs have developed rapidly,and tens of thousands of MOFs materials have been reported.Due to their advantages such as high porosity,easy functionalization and clear relationship between structures and properties,MOFs have shown enormous potential in gas adsorption and separation.However,due to the limitation for the types and quantities of metal ions and organic ligands in the synthesis process,the simple structures of MOFs are still difficult to meet the requirements of high adsorption capacity and/or high selectivity for specific gases in terms of structural accuracy and stability.In order to establish the structure-performance relationship and improve the gas adsorption and separation capability of MOFs,the synthesis of MOF materials has gradually developed from experimental exploration to precise design,including function-oriented pore structure design and optimization,pore tailoring,modification of secondary building units,etc.Based on the compositional characteristics of MOF materials and the concept of reticular chemistry,the construction of MOF materials with novel structure and functional pore environment have attracted scientists'attention by designing and introducing multiple structural components into one reticular structure.Reticular chemistry can realize the diversity of node types and connection modes through precise design,and effectively expand the topology of the skeleton and its functions;it can also obtain new MOFs with multifunctional and specific pore environments by introducing multicomponents into one structure.Besides,the materials can exhibit novel properties and variable functions due to the synergistic effects between multicomponents.In addition,isoreticular chemistry shows how to extend the structures and functions by adjusting the nodes while maintaining the integrity of the parent structure,and provides opportunities for the precise regulation of pores.Based on the parent structures of MOFs,isoreticular chemistry can perform further structural optimization in terms of adjusting ligand size,ligand functionalization modification,mixed ligands,and metal ion modulation to achieve more effective gas storage and separation.In this thesis,a series of novel MOF materials are synthesized based on the multicomponent strategy and the concept of isoreticular chemistry from metal ions and ligands.The applications for the as-synthesized MOFs in iodine adsorption,gas adsorption and separation are explored respectively.The multicomponent microporous MOF materials with excellent adsorption and separation properties in this study,provide valuable guidance for enriching the strategy to construct stable MOF materials.The thesis mainly includes the following three aspects:(1)By using the mixed ligand strategy,non-equilateral triangular pore microporous MOF materials can be constructed.Through appropriate functional modification of organic ligands,the structural stability can be enhanced,and the interaction between gas molecule and frame structure can be increased,the adsorption/separation performances of MOFs can be significantly improved.Herein,Cd(NO3)2·4H2O was chosen as metal source,ligands with different sizes were selected to synthesize target structures with optimized pore size and pore environment by functional modifications.Three ultramicroporous Cd-MOFs(compound 1-3)were successfully synthesized and their gas adsorption and separation abilities were systematically investigated.Compound 1 with non-equilateral triangular windows was constructed by two linear ligands with different length.Considering that the small window size and pore volume of compound 1 would limit its gas adsorption capacity,compound 2 with larger pore size and porosity was obtained by changing the length of ligands and expanding the original triangular pore while retaining the parent structure.At the same time,in order to enhance the stability of the framework and further improve the gas adsorption and separation performances of MOF,the ligand was modified with methyl group,and the pore channels were adjusted based on the frame structure of compound 2.The presence of methyl group not only accurately limits the pore window size,but also enhances the chemical stability and moisture stability.With the change of ligands length and substituent,the pore size in the structure is gradually optimized,and the adsorption capacity of C2H2 and the selective separation performance of C2H2/C2H4 are increased.Compound 3 showed better capture capacities of C2H2 and other small molecule gases than compound 1 and 2,and exhibited higher selectivity of C2H2/C2H4.Theoretical calculation further confirmed that suitable pore size and the presence of methyl groups could generate stronger host-guest interaction between compound 3 and C2H2,leading an excellent separation performance of C2H2/C2H4.This work provides a new idea for the construction of efficient gas separation materials from design to application.(2)In the preparation of mixed-ligands MOFs,organic ligands with different shape or coordination modes are conducive to the orderly arrangement of components in the structure,and ensure the distinctiveness and coordinations among the multiple ligands.Cu²⁺has various coordination modes and can coordinate with O,N and other electron-donor atoms to form mono-,bi-or multinuclear SBUs.Herein,Cu²⁺was selected with linear N-ligands and carboxylate ligands to construct three novel mixed-ligands Cu-MOFs(compound 4-6),and their applications in I2 adsorption and gas adsorption and separation were explored,respectively.Compound 4 is pillared-layer structure consisting of the one-dimensional(1D)Cu metal chains,the linear ligands and formic acid.The linear ligand and 1D Cu metal chain form a 2D layer structure rich in?-electron 1D straight channels,and then the adjacent layers are connected and supported by formic acid to form the final 3D skeleton.Compound 4 exhibited excellent adsorption and release properties of I2 due to the high?-electron-rich pore environment and high porosity.Compound 5 consists of two different inorganic secondary building units with linear N-ligands and triangular carboxylate ligands.Among them,the binuclear Cu cluster forms a pillared-layer structure with the linear ligand,while the tetranuclear Cu cluster forms a 3-fold interpenetrated layer structure with the triangular carboxylate ligand.Then the two types of layers are connected by tetranuclear Cu clusters to construct the final hierarchical structure-mimicking distribution structure.Compound 5 exhibited good water stability and moisture environments,and the synergistic effect of complex pore structure and abundant active sites significantly improved CO2 capture capacity.Compound 6 shows pore space partition structure,in which the linear ligand and the one-dimensional Cu chain form a three-dimensional skeleton with mesopores,and the triangular carboxylic acid ligand acts as a splitting ligand to divide the pore channel into two types of ultramicroporous channels.Due to the sieving effect of pore size,compound 6 shows purification ability of natural gas.Meanwhile,compound 6 has good acid-base and water stability.The successful synthesis and good adsorption properties of compounds 4-6 have confirmed that organic ligands with different shapes and coordination modes can not only construct MOFs materials with novel structure,but also improve the functionality of the skeleton,which shows a better application prospect in the field of adsorption and separation.(3)The diversity of conformations of the flexible ligands and the coordination preference with metals or metal clusters can be applied to synthesize MOFs with more diverse structures.However,due to the strong,how to use the flexible ligands to construct stable MOF materials is still the key point.Based on the Hard-Soft-Acid-Base theory,when the ligand and the coordination environment remain constant,high-valent metal ions with high charge density can form stronger coordination bonds,resulting in a more stable structure.Based on this,we successfully synthesized three stable MOF materials(compound 7-9).Compound 7 and compound 8 are iso-structural compounds consisting of 1D In chains,both of which are anionic frameworks.Compound 7 was prepared by In Cl3·6H2O and flexible ligand H2tazba.Among them,the terminal group coordination Cl^-of In³⁺,and a large number of dimethylamine cations with balanced charge in the channel which are difficult to remove,and affects the gas adsorption performance.Subsequently,In(NO3)3·4H2O was selected to construct compound 8and the original terminal group coordination of In³⁺was replaced by O,which reduced the electronegativity of the skeleton and made it easier to fully activate.Compound 8 showed high CO2 and Xe adsorption capacity.Compound 9 is a Zr-Cu bimetal-mixed MOF.According to the Hard-Soft-Acid-Base theory,Zr⁴⁺was coordinated with the carboxylic acid group in H2tazba ligand to form a three-dimensional structure,and Cu²⁺is coordinated with the N site in the ligand to immobilize the ligands at both ends of the pore channel,thus improving the stability of the skeleton.The introduction of the bimetal makes compound 9 exhibit good CO2adsorption ability and stability in acid and aqueous solutions.The successful synthesis of compounds 7-9 provides a new idea for the development of MOFs materials with diversified structure and strong stability by using flexible ligands.