Synthesis And Catalytic Property For CO₂ Cycloaddition Of MOFs With Multi-active Sites

The burning of fossil fuels has led to a rise of CO₂ in the air,resulting in serious environmental problems.In order to decrease the CO₂ concentration,discovering functional materials has become one of the effective methods to capture,transform and utilize CO₂.As a new kind of porous materials,metal-organic frameworks(MOFs)have shown significant performances in many fields.The advantages of MOFs like high surface areas,various topologies and flexible ligand functionalization also enable them with excellent performance in CO₂ adsorption,conversion and utilization.Among related studies,cycloaddition reaction of catalyzing CO₂ with epoxides into cyclic carbonates using MOF-based catalysts is one of the efficient CO₂fixation methods.Its atom-economy fulfills“Green Chemistry”and our country's“Carbon Peaking”and“Carbon Neutrality”goals.Besides,the cyclic carbonates are important chemical products in industry.Many researches about MOF-based catalysts for CO₂ cycloaddition reactions have been reported,but there still exists remained problems:(1)Few works systematically investigated the effect of active sites;(2)Studies about structural differences and influences on catalysis are not clear;(3)Strategies on synthesizing MOFs with significant catalytic efficiency are not enough.In order to solve the problems,we carried out a series of experiments and systematically explored them.In this thesis,modulator synthesis method and solvothermal synthesis strategy are used to construct new MOFs and explore their catalytic property for CO₂cycloaddition reaction.This thesis mainly includes the following three aspects:(1)Three analogous ligands with Lewis base sites(LBS)were selected to synthesize four reported isomorphous copper polyhedron-based MOFs(Cu-PMOFs),[Cu₆(tadipa)₃(Dabco)(H₂O)₂(DMF)₂]·13H₂O·7DMF(compound1),[Cu₆(tadipa)₃(H₂O)₆]·16H₂O·8DMF(compound2),[Cu₆(tpta)₃(DMA)₄(COO)(H₃O)]·12H₂O·7DMA(compound3)and[Cu₆(cbda)₃(Dabco)(H₂O)₂(DMF)₂]·8H₂O·3DMF(compound 4).Due to different organic groups and the existence of axial ligand,the four Cu-PMOFs possess different amounts of Lewis acid site(LAS)and different LBS types,leading to their distinct catalytic performances in CO₂ cycloaddition reactions.Compared to the other three MOFs,compound 2,which possesses the most LASs and an appropriate alkaline strength of the LBS,displayed the highest catalytic efficiency.Limited by pore sizes,compound 2 showed catalytic selectivity for small size epoxides.At the same time,it exhibits high catalytic efficiency in simulated post-combustion flue atmosphere.Recycling tests indicate that compound 2 has good recyclability and chemical stability after 5-time circulation experiments,proving the applicable values for compound 2 in the field of industrial waste gas purification.(2)Analogous V-shaped ligands were assembled with Zr₆ clusters to synthesize two novel Zr-MOFs[Zr₆O₄(OH)₄(obba)₆(DMF)₂]·4DMF(compound 5)and[Zr₆O₈(sdba)₄(H₂O)₈]·5DMF(compound 6).For comparison,another reported V-shaped ligand-based Zr-MOF[Zr₆O₄(OH)₄(dcpb)₆]·H₂O·2DMF(compound 7)is also constructed to systematically investigate the structure-property relations.The three ligands possess distinct sizes,torsion angles and functional groups,leading their differences in structures and properties.For structures,compound 5displays a 3D non-interpenetrated framework with pcu topology,compound 6 shows a2D sql structure while compound 7 exhibits a 3D double-walled interpenetrated framework with pcu topology.For catalytic property in CO₂ cycloaddition reactions,both compound 5 and 6 have LAS.Combined with their LBSs,the synergistic effect from multi-active sites endows compound 5 and 6 with significant catalytic efficiencies under high pressure conditions.However,the pore sizes and LAS amount of compound 5 is smaller than those of conpound 6,leading to a better catalytic efficiency for compound 6 than 5.Compound 7 has no active sites,leading to its poor catalytic efficiency.For gas adsorption/separation capabilities,due to the small pore sizes and lack of functional sites,compound 5 displays a low CH₄ uptake and significant C₃H₈/CH₄ selectivity.The sulfone group in compound 6 forms?-H interaction with C₂H₂,resulting in its highest C₂H₂ adsorption and best C₂H₂/CH₄separation performance among the three Zr-MOFs.Compound 7 has good gas adsorption capacity,but its gas selectivity is not prominent due to the lack of functional sites.Above experiments provide a feasible strategy for structural regulation and property enhancement of MOFs.(3)Different secondary building units and a nitrogen-rich ligand were assembled to form four reported MOFs[Cu₃(tatab)₂]·7.5H₂O(compound 8),[Zn₄O(tatab)₂]·3H₂O·17DMF(compound 9),[In₃O(tatab)₂(H₂O)₃](NO₃)·15DMA(compound 10)and[Zr₆O₄(OH)₇(tatab)(Htatab)₃(H₂O)₃]·x Guest(compound 11).The four compounds all contain LASs and LBSs.Besides,the large pore sizes of compound 8 enhance the concentration of CO₂ and epoxides,which endow it with the best catalytic performance among the four compounds.Meanwhile,the data proved that Cu-paddlewheel(compound 8)and Zn₄O(compound 9)have better catalytic performances than In₃O(compound 10)and Zr₆(compound 11).But for Cu-paddlewheel and Zn₄O,the different coordination modes limit the comparison between compound 8 and 9.Further tests indicate that compound 8-11 display better catalytic performances than the other seven,which prove that the strategy of introducing multi-active sites into MOFs to improve the catalytic property of CO₂cycloaddition reactions is feasible.It also gives guidance to construct MOF-materials with high catalytic efficiency.