Construction Of Zn-MOFs Based On Nitrogen Heterocyclic Carboxylate Ligands And Their Applications In Carbon Dioxide Conversion

CO₂ as the main greenhouse gas has caused global warming,causing a series of environmental disasters,and even threatening the survival of human beings.However,CO₂is still a cheap and available chemical raw material.At present,CO₂capture and conversion are of great strategic significance for CO₂emission reduction and resource utilization.In this field,the preparation of cyclic carbonates by cycloaddition reaction of epoxides and CO₂is undoubtedly the most promising development direction,because the reaction meets the sustainable principle of green chemistry,and the atomic economy reaches 100%.However,CO₂is an inert molecule,it is very important to find suitable catalysts for CO₂conversion.Among them,metal organic frameworks(MOFs)are a class of excellent heterogeneous catalysts.They not only have high specific surface area and porosity,but also can be designed and modified in advance to achieve controllable adjustment of structure and performance.Nevertheless,the synthesis and application of MOFs are facing the problems of low stability,less active sites and poor catalytic activity,which limit the development of MOFs to a certain extent.Therefore,the preparation of MOFs with excellent performance is of great significance for CO₂conversion.Based on the reported mechanism that zinc centers in MOFs could form Zn-O adducts with the oxygen of epoxides to activate the epoxides in the process of CO₂conversion,a three-dimensional Zn(Bmic)(AT)MOF catalyst with Lewis acid-base sites was prepared by solvothermal method using mixed ligands of1H-benzimidazole-5-carboxylic acid and 5-aminotetrazole.Rich amine groups in porous Zn(Bmic)(AT)contributed to CO₂adsorption,and the CO₂adsorption capacity was 79 cm³/g at 273 K and 52 cm³/g at 298 K,respectively.In addition,the Lewis acid-base sites and porous structure of Zn(Bmic)(AT)could promote the conversion of CO₂with various small sized epoxides into cyclic carbonates together with the Bu₄NBr cocatalyst,reaching a high TON(170)for propylene carbonates at80 ^oC,0.5 MPa and 6 h.Besides,Zn(Bmic)(AT)catalyst exhibited a good recyclability for six consecutive cycles.Additionally,the changes of ligand coordination sites and coordination modes will influence the structure of the prepared MOFs,resulting in the great differences in functions.On the basis of the MOF designed in the previous chapter,a novel zinc based MOF catalyst with high coordination and stability was synthesized by changing the structure of the main ligand,replacing the benzene ring structure with a heterocycle,and introducing carboxyl group into the para position.The catalyst was composed of mixed ligands of 3,5-pyrazole dicarboxylic acid and5-aminotetrazole,which had available Lewis acid-base sites.The presence of Lewis base sites could increase the affinity for CO₂,thus giving Zn(PZDC)(ATZ)a higher CO₂adsorption capacity of 60.2 cm³/g(273 K,1 atm)and 48.7 cm³/g(298 K,1 atm),respectively.In addition,Zn(PZDC)(ATZ)/Bu₄NBr catalyst system promoted the preparation of cyclic carbonate from various epoxides substrates and CO₂under solvent-free conditions(90 ^oC,1 MPa,6 h),in which the TON of propylene carbonate could reach 131,and the catalyst showed a steadly activity after 7 cycles.Furthermore,the structure of Zn(PZDC)(ATZ)remained stable in acid-base and water environment.In order to achieve the high catalytic activity of MOF catalyst for large-scale epoxides,a new tetratopic bridging ligand 1,1'-(propane-1,3-diyl)bis(1H-pyrazole-3,5-dicarboxylic acid)(PDC)was synthesized,and the crab-like structure of Zn-2PDC was synthesized by solvothermal reaction.By vacuum drying of Zn-2PDC,abundant open metal sites(OMSs)generated after the removal of coordinated water molecules,which produced exceptionally catalytic performance for CO₂cycloaddition to bulky styrene oxide,and the TON of styrene carbonate could reach up to 171 under mild conditions of 55 ^oC,12 h and 1 MPa.Moreover,MOF Zn-2PDC also showed a generality to other bulky epoxides.Meanwhile,the MOF Zn-2PDC catalyst remained good catalytic performance after six cycles.This thesis not only explored the effects of carboxylic acids,metal centers and flexible ligands on the structure of MOFs,but more importantly,the synthesized catalysts had excellent performance in the catalytic conversion of CO₂,which provides a valuable reference for the future scientific research on the construction of MOFs for CO₂ conversion.