Study On The Construction Of Heterometallic Cluster-based MOFs And Their Catalytic Performance For CO₂ Cycloaddition Reaction

As a kind of abundant,cheap,environmental friendly and renewable C1 resource,the resource utilization of CO₂can not only reduce the negative impact of its emission on the environment,but also be applied to related industries as an important energy product to reduce energy consumption.The cyclic carbonate produced by cycloaddition reaction of epoxide and CO₂is an ideal"green chemistry"product,which is used in the synthesis of fine chemicals intermediates,polymer materials synthesis precursors and lithium battery electrolytes.Nevertheless,because of the thermodynamic stability and kinetic inertia of CO₂,high energy consumption is usually required when it is converted into high value-added products through organic catalytic reactions.Moreover,high temperature and pressure conditions required for industrial synthesis increase the reaction energy consumption and product cost.Therefore,the key to solve the problem of catalysis lies in the rational design of efficient and environmental friendly catalysts.Metal-organic frameworks（MOFs）are considered as potential catalysts for CO₂cycloaddition due to their excellent structural properties and unique confined pore environment.At present,most MOFs catalysts are used to catalyze the cycloaddition reaction of epoxides with CO₂using single metal node MOFs or mixed metal-organic frames（MMOFs）as catalysts.In contrast,hetero-metal organic frames（HMOFs）are self-assembled from at least two metal and organic ligands,and the synergistic effect between hetero-metal sites can significantly improve catalytic efficiency.HMOFs greatly enrich the possibility of metal cluster assemblage and provide a clear structure composition and catalytic center,which makes it possible to study the structure-activity relationship between structure and catalytic performance.However,HMOFs also put forward higher requirements on the self-assembly environment,so the research of HMOFs as catalyst for cycloaddition reaction is very limited.Based on this,three similar structures of nitrogen-containing polycarboxylic acid ligands were designed and synthesized in this paper.Four HMOFs with novel structures were obtained by solvothermal synthesis of these ligands and metal salts.Finally,the performance of HMOFs on CO₂cycloaddition reaction is systematically studied and compared,and the main research contents and conclusions are as follows:（1）Based on the pentaacid nitrogen-containing aromatic ligand 2,6-bis（2,4-dicarboxylphenyl）-4-（4-carboxylphenyl）pyridine（H₅BDCP）,two 3D porous HMOFs materials were constructed,{[（CH₃）₂NH₂]₂[Sn Yb₂（BDCP）₂]·2DMF·5H₂O}_n（NUC-30）and{[In₂Tm₂（BDCP）₂（μ₂-OH）₂（H₂O）₂]·4DMF·3H₂O}_n（NUC-56）.The HMOFs of{[（CH₃）₂NH₂]₂[Ba Zn（TDP）（H₂O）]?DMF?5H₂O}_n（NUC-51）was constructed from the hexaacid nitrogen-containing aromatic ligand 2,4,6-tris（2,4-dicarboxylphenyl）pyridine（H₆TDP）.Based on 2,4-di（2-amino-4,6-dicarboxyphenyl）pyridine ligand（H₆TDP-NH₂）,a three-dimensional heterometallic HMOF{（Me₂NH₂）[In Tm₂（TDP-NH₂）₂]?3DMF?4H₂O}_n（NUC-16）was constructed.The results of single crystal diffraction show that the HMOFs presented three-dimensional porous structures containing heterogeneous metal clusters,and the porosity of all HMOFs exceeds 45%.In addition,all HMOFs have nanoporous channels with larger pore sizes,good structural stability and abundant Lewis acid-base reaction active sites,which lay a good foundation for cycloaddition catalysis.（2）First,the optimal reaction conditions of four HMOFs were investigated by controlled variable experiments using styrene oxide and CO₂as models and tetrabutylammonium bromide（n-Bu₄NBr）as cocatalyst.At the same time,the catalytic stability,recyclability and heterogeneity were tested.The results showed that all HMOFs showed high cycloaddition catalytic efficiency（yield not less than 95%）,and good heterogeneity and recyclability（more than five cycles）under appropriate mild conditions（e.g.,temperature not higher than 75°C,pressure not more than 1atm,time within 8h）.Secondly,using n-Bu₄NBr as cocatalyst,the cycloaddition reaction performance of HMOFs on CO₂and epoxides containing different substituents was explored.The results showed that HMOFs showed better catalytic universality.With the increase of steric hindrance of epoxide substituents,the yield of target products only decreased slightly（from 100%to 95%,within 5%）.Thirdly,the catalytic mechanism of the cycloaddition reaction catalyzed by HMOFs was discussed.The catalytic mechanism is similar to that of monomeric MOFs,but the difference is that the metal sites of HMOFs can provide more abundant catalytic sites for the activation of epoxides,thus promoting the reaction.Finally,by comparing the relationship between the structure of the HMOFs catalyst prepared in this paper and the catalytic performance of cycloaddition,it was found that large pore size and high porosity,appropriate acidity of heterometal cluster groups and ligand alkaline catalytic sites,as well as the stability of HMOFs framework,these factors are the key to promote cycloaddition reaction.This work provides a useful idea for HMOFs catalyzed cycloaddition reaction.