Functionalization Of MIL-101 For CO₂ Cycloaddition Catalysis

CO2-epoxide cycloaddition(CCE)represents one of most attractive approaches for chemical fixation and utilization of CO2,but highly efficient and fully heterogeneous green catalysis is still challenging.The free nucleophilic anions required for efficient catalysis are not easy to heterogenized,and the use of homogeneous halides ions lowers the economy and benignity.Ionic metal-organic framework(IMOFs)having cationic frameworks with "free" anions inside pores are nice porous matrices heterogenizing the essential nucleophilic anions and meanwhile providing Lewis acids.However,the catalytic activity has been limited for low halogen anion content and lack of new metal centers or auxiliary catalytic sites.This dissertation is devoted to catalysis-targeted design and preparation of the IMOFs by means of the facile and widely applicable Cu(I)-catalyzed azide-alkyne cycloaddition(CuAAC)and N-alkylation reactions.Systematic and effective methods for the ionization and multifunctionalization of MOFs were established.New functional sites were introduced through ionization,metallation,and introduction of hydrogen bond donor.The structure-activity relationship between catalytic performance and functional units was explored to gain an in-depth understanding of the IMOF-catalyzed CCE reaction.Green heterogeneous catalytic systems were obtained for facile and efficient conversion of CO2.The research content of this paper is mainly divided into the following three parts:1.Quaternary ammonium functionalized triazole-type IMOFsA series of IMOFs with high halide content were facilely prepared by reacting azide-tagged MIL-101 with alkyne-tagged organic bromide salts.The efficient azidealkyne click reaction within the mesoporous MOF allows quantitative cationization of the linkers,and the resultant bifunctional IMOFs contain one halide ion per metal center to the benefit of the one-to-one cooperation between the nucleophilic and Lewis-acid.With these IMOFs,the effects of different cationic groups and halide anions on the catalytic reaction within the confined ionic space were systematically studied.We revealed that the effects within the confined ionic space are different from those for homogeneous halides.For IMOFs,a small cationic group is beneficial for catalysis,and the promotion by halide ions increases as Cl-<I-<Br-.The tendencies could be ascribed to the compromise between intrinsic nucleophilicity of the anions and the pore size effects.The optimal catalyst,MIL-101-tz-TMA-Br,outperforms previous IMOF catalysts.The catalyst works under mild conditions and shows excellent recyclability.2.Metal-functionalized triazolium-type IMOFsIMOFs with multiple catalytically active sites were prepared by ionization and metallation modification.1)Hydrogen bond donor were introduced through the CuAAC modification of MIL-101-N3 with functionalized alkynes.2)Nucleophilic anions and more hydrogen bond donor were introduced through N-alkylation of triazole rings.3)New metal sites were introduced through the chelating coordination of triazole and carboxyl groups.Using these MOFs catalysts,the effects of different functional groups,different metal Lewis acid sites,and nucleophilic bromide ions on the catalytic reaction were systematically studied.Studies have shown that the promotion effect of carboxyl group is greater than that of alkyl group,hydroxyl group and amino group.The introduction of different metal centers can significantly improve the catalytic activity,and Zn2+promotes the catalytic reaction more than Cu2+,Mn2+ and Co2+.The obtained multifunctional IMOF catalyst MIL-101-TzmCOOH-Br@ZnBr2 exhibits excellent catalytic activity,which is superior to that of previously reported IMOF catalysts,and is reusable and has good universality.3.Synthesis and catalytic properties of hydroxyl-functionalized triazolium-type IMOFsIn this chapter,the previous ionization and functionalization strategies are combined to further improve the catalytic performance.1)A series of mono-cationic IMOFs with quaternary ammonium grafted at linkers and bromide anions inside pores were prepared by facile click chemistry.2)Different numbers of hydrogen bond donor(hydroxyl groups)were introduced in ionization step for activity optimization.3)To introduce more anions,bi-cationic IMOFs were created by a second ionization step,Nmethylation at triazolyl to create triazolium.With the stepwise ionization and functionalization,we demonstrated the synergic catalytic effects of bromide with metal centers and hydroxyl groups,the impacts of hydroxyl/bromide content,and the size effect of the cationic group.Introducing more hydroxyl groups can enhance the activity,but the effect can be overcompensated by the negative steric effect of bulky groups.Creating new cationic groups by N-methylation at existent groups allows the inclusion of more bromide anions without much steric cost,so the bi-cationic IMOFs showed significantly enhanced activity.The optimal mono-cationic and bi-cationic catalysts outperforms previous ionic heterogeneous catalysts based on MOFs,silica,and various organic polymers.In particular,the bi-cationic catalyst can quantitatively convert epichlorohydrin to the carbonate within 1 h at 1 MPa CO2 and 80℃,the apparent turnover frequency being up to 478 h-1 in 15 min.The highly active and selective,fully heterogeneous and recyclable catalyst has the appeal for practical applications.