Hydrogen Storage And Release With CO₂ Catalyzed By Functionalized Amido Iridium Complexes

Extensive consumption of non-renewable fossil resources by human results in a large amount of CO₂ emission and serious environmental pollution.Therefore,the search for renewable green energy is a hot spot of recent energy development.Hydrogen,as an emerging green energy source,has the advantages of high combustion calorific value,environmental friendly and no pollution,and has attracted widespread attention.However,hydrogen is gas at room temperature and is flammable and explosive,which making the storage and transportation more difficult.How to achieve safe and efficient storage and transportation of hydrogen is the key to the development and utilization of hydrogen.Formic acid is a promising chemical hydrogen storage material,which has the characteristics of thermodynamic stability,liquid at room temperature,and high hydrogen storage density（4.4wt%）.Moreover,formic acid can be prepared by CO₂ hydrogenation,which has high environmental benefits and good reproducibility,make it becomes a hot topic in the field of hydrogen storage and release.In this thesis,a series of functionalized amido iridium complexes with basic pendant groups were designed and synthesized,which were used in the homogeneous catalysis of CO₂hydrogenation to formic acid and formic acid dehydrogenation in the aqueous phase.We tested their catalytic performance and explored the catalytic reaction conditions and catalytic reaction mechanism.（1）A series of amide ligands and their iridium complexes were designed and synthesized,and the structures were characterized by NMR and other means.（2）The performance of these iridium complexes in catalyzing the CO₂ hydrogenation reaction was tested.The cholro iridium catalysts[Cp*IrLCl]Cl with better catalytic activity were identified and their corresponding hydrates[Cp*IrL（H₂O）]SO₄,which have better water solubility,were synthesized.It was found that increasing the water solubility of the catalyst had no significant effect on the CO₂ hydrogenation process.（3）The performance of this series of iridium complexes in catalyzing the dehydrogenation of formic acid was tested.The corresponding hydrates[Cp*IrL（H₂O）]SO₄,with better water solubility were found to be highly active.The effects of reaction conditions such as pH,temperature,and catalyst loading on catalytic performance of formic acid dehydrogenation were explored using corresponding catalyst hydrate.Through a comparison,the catalyst Cat7d with N-（4-hydroxyphenyl）picolinamide showed the best activity.At 60°C,pH 2.81,the initial turnover frequency（TOF）reached 73 269 h^-1.It is one of the catalysts with the excellent catalytic activity reported so far.（4）The catalyst Cat7d was used to explore the reaction mechanism.The formation of Ir-[H]intermediate was detected.And the kinetics isotope experiment（KIE）was performed at the same time.It was found that catalyst Cat7d had different rate determination steps under different pH conditions:when pH=1.84-2.81,the elimination ofβ-H is the rate determining step;when pH>2.81,the generation of H₂ is the rate determining step.Final,we proposed a possible reaction mechanism.In this study,we designed and synthesized a series of new amido iridium catalysts,which showed good catalytic activity for CO₂ hydrogenation,and exhibited excellent catalytic activity for formic acid dehydrogenation.The reaction mechanism was explored andprovided insightful informations for the design of novel catalysts.