Construction Of Functional Metal-organic Frameworks And Their Applications In The Separation And Electrocatalytic Reduction Of Carbon Dioxide

Anthropogenic carbon dioxide emissions from fossil fuel combustion have become a crucial factor of global greenhouse effect,which result in a series of environmental issues.Meanwhile,carbon dioxide is also a renewable and abundant carbon source.Therefore,efficient separation,capture and catalytic conversion of CO₂ is an important pathway to alleviate the global energy and environment crisis.Metal-organic frameworks（MOFs）are a class of porous crystalline materials constructed by assembling metal ions/clusters and organic linkers through strong coordination bonds.The abundance of metal ions/clusters and the versatile geometries of organic ligands endow MOFs with extraordinary structural merits,such as ultrahigh porosity,large specific surface area,tunable aperture structure,and diverse functionalities,which underpin their widespread applications in CO₂ separation and storage,and catalytic conversion.For instance,the development of MOFs-based mixed matrix membranes（MMMs）can effectively eliminate the“trade-off”effect of conventional polymer membranes by integrating the molecular sieve effect of MOFs with the low cost and easy processability of polymer matrix.However,the practical application of MOFs-based MMMs is still hindered due to the poor interfacial compatibility between MOFs and polymer matrix,as well as the undesired sedimentation of MOFs particles.Meanwhile,MOFs are also widely investigated in the field of electrochemical reduction of CO₂ due to the high specific surface area and various active sites.However,the catalytic activity of MOFs is always limited because MOFs themselves are not good electron conductors.As a result,the construction of MOFs materials with desired functional properties is of great significance to achieve efficient CO₂ separation and capture,as well as effective CO₂ conversion.To overcome abovementioned problems,this thesis has carried out the following experiments:（1）To conquer the limitation of poor interfacial compatibility between MOFs phase and polymer phase in MOFs-based MMMs,we have designed and synthesized a new organic linker containing vinyl groups,and constructed BUCT MOFs with polymerizable moieties.MMMs with enhanced MOFs/polymer interfacial adhesion were then prepared via the copolymerization of polymerizable BUCT MOFs with organic monomers,that enabled good separation efficiency of CO₂.BUCT MOFs exhibited high selectivity towards CO₂,and their double bonds revealed good reactivity.Specifically,BUCT-2-based MMMs showed the best gas separation performance with a CO₂ permeability of 635.1 barrer and a CO₂/N₂ selectivity of 41.8.Dramatic 150.6%and 142.1%improvements in CO₂ permeability were obtained in BUCT-2-based MMMs compared with the pristine PEO membrane.The relevant CO₂/N₂ selectivity showed125.4%and 122.3%enhancements in comparison with its counterpart.This strategy extends the methods in the preparation of MOFs-based MMMs and provides a new avenue for the application of polymerizable MOFs in gas separation.（2）To circumvent the limited electrical conductivity of conventional MOFs,we have designed and synthesized a new AQM-H₂L carboxylic acid ligand with redox reactivity using para-Azaquinodimethane（p-AQM）as the skeleton.A series of novel MOFs with low band gap were successfully constructed using zinc,copper and bismuth as metal nodes,and the electrochemiacal performance of CO₂ reduction was studied.The three MOFs are named as AQM-Zn MOF,AQM-Cu MOF and AQM-Bi MOF,respectively.Their structures were successfully analyzed by single crystal X-ray diffraction.The energy band gap of AQM-Cu MOF is 1.94e V,and the LUMO is-2.12 V（vs Ag/Ag Cl）,which is more negative than the redox potential of-0.48 V（vs Ag/Ag Cl）rewuired for CO₂ conversion to formic acid.These results indicated that the electrochemical reduction of CO₂ to formic acid by AQM-Cu MOF is feasible on the thermodynamics.To further enhance the electron transfer efficiency of AQM-Cu MOF,the strong electron acceptor molecule TCNQ was successfully doped into AQM-Cu MOF,and the basic structural characterization was completed.In addition,to improve the stability of MOF material,we also constructed AQM-Bi MOF and investigated its electrochemical performance in CO₂reduction.The faraday efficiency for the production of formic acid from CO₂ can reach 76%.AQM-Bi MOF also displayed good electrochemical stability.（3）To improve the electron transfer efficiency within MOFs,it is expected to design organic ligands that can generate free radicals during electrochemical reactions.Therefore,we demonstrated for the first time the synthesis of novel nitrogen hybridization ionic molecules using the facile nucleophilic N-substitution reaction between pyridyl nitrogen and the AQM ditriflate.The results elucidated that these i AQM-viologens had strong electron accepting abilities.Spectroelectrochemical study showed that the i AQM-BPY²⁺and AQM core free radicals were produced during the two-step reduction process of i AQM-BPY⁴⁺-Et OH.Meanwhile,the 1:2supramolecular complex of i AQM-BPY⁴⁺-Et OH and cucurbiturone（CB[7]and CB[8]）was constructed by self-assembly,and its structure was characterized by single crystal X-ray diffraction.This chapter plays an important predictive role for further construction of functional organic ligands and MOFs that can generate stable free radicals to enhance the electrochemical catalysis.