MOF Supported Ru-based Heterogeneous Catalysts For Catalytic Hydrogenation Of Carbon Dioxide To Formic Acid

The catalytic conversion of CO₂ can not only alleviate the greenhouse effect caused by excessive CO₂ emissions in the atmosphere,but also produce high value-added chemicals and fuels.The catalytic hydrogenation of CO₂ to formic acid is an atom-economic reaction,and formic acid is not only a basic organic chemical feedstock widely used in various industrial fields,but also is considered as one of the most promising hydrogen storage materials.In this thesis,MOF-supported Ru-based heterogeneous catalysts designated as Ru-NHC-MOF and Ru Ag@MIL-101 for catalytic hydrogenation of CO₂to formic acid were investigated.A series of different Ru-NHC-MOF heterogeneous catalysts were synthesized by immobilization of Ru Cl₃,[Ru（C₅Me₅）Cl₂]₂（C₅Me₅=pentamethylcyclopentadienyl）,[Ru（C₆Me₆）Cl₂]₂（C₆Me₆=hexamethylbenzene）on a N-heterocyclic carbene based MOF through liquid impregnation method.The Ru3-NHC-MOF catalyst prepared by the immobilization of[Ru（C₆Me₆）Cl₂]₂exhibited the highest catalytic activity because of its strongest electron-donating ability of C₆Me₆ligand of[Ru（C₆Me₆）Cl₂]₂complex,thus elevating the nucleophilicity of Ru and promoting dissociative adsorption of H₂and the formation of Ru-H bond.The use of inorganic salt additive and strong polarity solvent would reduce the activation barrier of CO₂ insertion into the Ru-H bond,thereby significantly enhancing catalytic activity.A high TON of 3803 for Ru3-NHC-MOF catalyst was obtained at 120℃under a total pressure of 8 MPa（H₂/CO₂=1:1）for 2 h with K₂CO₃ additive in DMF solvent.A series of Ru Ag@MIL-101 heterogeneous catalysts with different molar ratios of Ru to Ag were prepared using MIL-101 as support which has large specific surface area and is stable in alkaline solution.When the molar ratio of Ru to Ag was 1:4,the Ru₁Ag₄@MIL-101 catalyst showed the highest catalytic activity.The Ru Ag nanoparticles were uniformly dispersed in MIL-101 support with an average particle size of 1.79 nm.The synergistic effect of Ru and Ag existed in bimetallic Ru Ag alloy nanoparticles.The ability of Ag atoms to donate electrons to Ru atoms after alloying accounted for enhanced catalytic activity.Addition of amount of water would remarkably enhance catalytic activity because the activation barrier of CO₂ insertion into the Ru-H bond was decreased in the presence of water.A high TON value up to16966 for Ru₁Ag₄@MIL-101 catalyst was achieved at 120℃under a total pressure of 8 MPa（H₂/CO₂=1:1）for 2 h with DMF and H₂O as mixed solvents,NEt₃ as base and K₂CO₃ additive,outperforming other heterogeneous systems reported in the literature.Furthermore,the Ru₁Ag₄@MIL-101 heterogeneous catalyst still exhibited favorable catalytic activity upon four catalytic cycles.