Structural Design And Catalytic Property Of Molecular-type Catalysts And Single-atom Catalysts For Carbon Neutralization

As a typical greenhouse gas with increasing concentration,carbon dioxide（CO₂）has a significant influence on global climate and atmospheric temperature.CO₂cycloaddition and Knoevenagel condensation are effective means widely adopted to mitigate environmental issues and improve energy efficiency.Either molecular-type catalysts or single-atom catalysts has homogeneous active centers,which make it easy to establish the adsorption relationship between the catalyst and the reaction substrate.It has attracted extensive attention to design efficient carbon neutral catalytic conversion materials.In contrast,other traditional catalytic systems are difficult to identify the active centers due to size effect,carrier effect,surface interface effect and many other influencing factors,which make the study of catalytic mechanism difficult.Therefore,molecular-type catalysts and single-atom catalysts are more effective in helping to understand the mechanisms of catalytic reactions.However,they both suffer from the problem of structural instability.Therefore,this paper enhances the stability and catalytic activity of the catalysts by changing the microenvironment（short-range and long range）of the active centers.The main strategies are the synthesis of low-dimensional molecular-type catalysts using ligand modulation and changing the metal site species,and the synthesis of single-atom catalysts using electronic metal-support interactions.Results show that these strategies can significantly improve the catalytic performance of Knoevenagel condensation reactions for CO₂ cycloaddition.The dissertation shows results as follow:（1）The effect of the adjustment of the active site microenvironment of the molecular type catalyst on the catalytic performance of the Knoevenagel condensation and the CO₂ cycloaddition reaction was investigated.Five low-dimensional complexes were synthesized by the ligand modulation strategy.It was found that adjusting the length of the molecular chains of the organic ligands could change the stability of the catalysts.Among them,the yield of styrene oxide was 100%for 19 h at room temperature and pressure for complex 2,and it could still reach 94%after five cycles;The number of metal sites could also be changed by ligand adjustment,and crystal structure tests showed that the number of metal sites in complex 5 was twice that of complex 4,which significantly improved the catalytic activity of the catalyst.（2）The effects of different metal types on the catalytic performance of CO₂cycloaddition in low dimensional complexes were investigated.Five single-core M（II）zero-dimensional complexes were obtained by self-assembly of molecular catalyst.It was found that the catalytic performance of the CO₂ cycloaddition reaction could be changed by adjusting the type of metal sites.As the active sites Mn（II）in complex 6has a strong Lewis acidity,the catalytic effect is obvious due to the complexes 7 and 8.Through the ligand adjustment strategy,it was found that the catalytic activity of the catalyst could still be improved,and the turnover frequency（TOF）of complex 9 was19 times that of complex 6.（3）The effect of spatial dimensionality of molecular-type catalysts on the catalytic performance of Knoevenagel condensation reaction was investigated.Different dimensional Cu complexes were synthesized by the in situ self-assembly of metal ions with organic ligands.Together with the zero-dimensional Cu complex 10 in the previous chapter,the catalytic performance study of the Knoevenagel condensation reaction was carried out.Ultrasound-assisted experiments verified that in situ catalysis is also efficient in catalytic performance.The experimental results show that the remote microenvironment of the active center is dimensionally altered by ligand modulation,which has a negative correlation with the catalytic performance.In combination with theoretical calculations,it was demonstrated that the decisive step of complex 10 in the Knoevenagel condensation reaction（room temperature,2 h,100%）is the departure of benzylidene malononitrile,with a reaction energy base of 73.2 k J mol^-1.（4）The effect of electronic metal-support interactions on the catalytic activity of single-atom catalysts in the CO₂ cycloaddition reaction.We developed Ir single atoms stably anchored WO₃ support（Ir₁-WO₃）with a strong electronic metal-support interactions.Supeirior CO₂ cycloaddition is realized in the Ir₁-WO₃catalyst via the electronic metal-support interactions effect:100%conversion efficiency for the CO₂cycloaddition of 1-phenyl-1,2-epoxyethane to styrene carbonate after 15 h at 40℃,and excellent stability with no degradation even after 10 reaction cycles for a total of more than 150 h.Density functional theory calculations reveal that the electronic metal-support interactions effect results in significant charge redistribution between the Ir single atoms and the WO₃ support,and consequently lowers the energy barrier associated with epoxide ring opening.