Synthesis And Application Of Proton Responsive Complexes For Catalytic CO₂ Hydrogenation

The application of fossil fuels has resulted in a large amount of the carbon dioxide emission, which causes serious greenhouse effect. Transformation of carbon dioxide into value-added chemical materials not only realizes the recycling and utilization of waste, but also reduces environmental pollution. The application of carbon dioxide shows important practical significance for serious environmental problem and energy issues. However, carbon dioxide transformation usually requires harsh reaction conditions for some selective reaction because of its high oxidation state and thermodynamic stability. Using transition metal complexes as catalysts can effectively lower the pressure and temperature. Among the numerous carbon dioxide transformations, catalytic hydrogenation of carbon dioxide in aqueous solutions is supposed to be a significant way, which is not only effective for production of chemical materials but also more environment friendly by avoiding utilization of organic solvent.In this thesis, a series of water-soluble metal complexes was synthesized for reductive hydrogenation of CO2 in aqueous solutions. The catalytic activity of complexes and reaction mechanism were explored.1) A series of water-soluble, proton-responsive Ir complexes coordinated by monodentate ligands were prepared. The pyridine ligands with hydroxy or sulfydryl group are commercially available and cheap. In addition, the synthetic method for metal complexes is simple and easy.2) The catalytic activity of the monodentate complexes with different ligands were screened for reductive hydrogenation of CO2 in aqueous KHCO3 solution. The effect of catalyst loading, pressure and ratio of H2 and CO2 on the reaction was examined. The optimized reaction conditions and catalyst were determined by comparing the experimental results. When the hydrogenation reduction reaction of CO2 was carried out by Ir complexes with 2-hydroxylpyridine at 5 MPa,100 ?,1:1 H2/CO2 (v/v) in 2 mol/L aqueous KHCO3 solution, TOF and TON (24 h) were obtained respectively 180 h-1 and 780.3) A mechanism for the hydrogenation reduction reaction of CO2 by proton-responsive Ir complexes with monodentate ligands was proposed according to experimental results. The CO2 insertion into Ir hydride should be the rate determining step, and the adjacent hydroxyl played an important role in the catalytic process.4) The Ir, Ru complexes coordinated by bidentate bipyridine with methylthio group were firstly prepared for hydrogenation reduction of CO2 in aqueous KHCO3 solution. The effect of temperature, pressure, and concentration of KHCO3 on the reaction was examined. The activation energy for hydrogenation reduction of CO2 by Ir, Ru complexes were calculated to explain that the higher reaction activity of Ir complexes than that of Ru complexes.This study developled a new type of water-soluble complexes for CO2 hydrogenation in aqueous solutions. The experimental results and analysis provide the theoretical basis for design of new catalysts for CO2 hydrogenation in future.