Preparation Of Acid-Base Bifunctional Metal-Organic Frameworks And Its Catalytic Performance In Cascade Reactions

Cascade reaction is a reaction process consisting of two or more continuous independent reactions and occurs in a reactor without separation and purification of intermediates.Therefore,cascade reaction can effectively simplify the process,reduce energy consumption,and is an efficient and environmentally friendly chemical synthesis method.The design of catalyst is the key step of cascade reaction because multiple catalytic active centers are used simultaneously to realize cascade reaction.Metal-organic framework is a kind of porous crystal material formed by coordination of metal ions or clusters and organic ligands.In addition to high porosity,high specific surface area and other characteristics,the metal organic frame is also characterized by easy functionalization,so it has strong designability.By designing different types of catalytic active sites(such as metal nodes,organic ligand catalytic active centers and catalytic species in pores)in the metal-organic framework,it is expected that the metal-organic framework can be made into a multifunctional catalytic material for cascade catalytic reactions.In this paper,a series of acid-base bifunctional metal organic frames were prepared under control.The model was acetal hydrolysis and aldehyde-nitrile condensation cascade reaction.By controlling the catalytic activity center,the catalytic performance of the acid-base bifunctional metal organic frames was adjusted.(1)The acid-base bifunctional metal-organic frameworks:Ui O-66-NH₂-SO₃H rich in amino and sulfonic acid groups was prepared by mixed ligand synthesis and post-modification synthesis.The bifunctional active centers of the catalyst were uniformly distributed in the metal organic framework.By controlling the proportion of mixed ligands,the content of catalytic active sites in the metal organic framework can be changed.Compared with the metal-organic framework containing a single active center(Ui O-66-NH₂and Ui O-66-SO₃H),Ui O-66-NH₂-SO₃H showed more excellent catalytic activity through the synergistic action of acid and base active centers,so that the reaction yield could reach 100%.When the relative ratio of the two active centers is changed,the yield of the reaction will change.Compared with the catalytic situation when the acid-base active center was 1:1,the overall reaction rate of the cascade reaction was improved when the acid active center was increased,and the yield of the cascade reaction with the 4-propyl benzaldehyde diethyl acetal and 4-(diethoxymethyl)-inverse-homo-stilbene as the substrates were also increased by 3.5%and 6%,respectively.Moreover,Ui O-66-NH₂-SO₃H showed good recyclability.After 5cycles,it still maintained good catalytic activity.(2)Two acid and base bifunctional metal organic frames of core-shell structures were synthesized by epitaxial growth method:Ui O-66-SO₃H@Ui O-66-NH₂and Ui O-66-NH₂@Ui O-66-SO₃H.The acid and base active centers were enriched in the shell layer or the core layer in an orderly manner,and the separation of the two catalytic active centers was realized.When applied to cascade catalytic reaction,the acid-base bifunctional metal-organic frameworks of core-shell structure showed dimensionally selective catalytic performance.As the substrate is more likely to spread in MOFs after hydrolysis,the overall catalytic effect of Ui O-66-NH₂@Ui O-66-SO₃H is obviously better than that of Ui O-66-SO₃H@Ui O-66-NH₂.The catalytic yield of Ui O-66-NH₂@Ui O-66-SO₃H is up to 100%and 92.2%.The size of 4-(diethoxymethyl)-inverse-homogeneous styrene is large,so it is difficult to diffuse in MOFs,and it is difficult to contact with the active sites of the core layer.Moreover,it is easy to block the MOFs channel and inactivate MOFs.Therefore,when Ui O-66-SO₃H@Ui O-66-NH₂and Ui O-66-NH₂@Ui O-66-SO₃H as catalyst,the reaction yield is only 53.6%and 72.4%,respectively,and Ui O-66-NH₂and Ui O-66-SO₃H catalytic results close,did not clearly show the advantages of acid and alkali active site synergy.