Hydrogen Storage And Release Using CO₂ Catalyzed By Iridium Complexes Bearing Pendent Group

At present,the large-scale use of fossil fuels has caused the CO₂ content in the atmosphere to continue to increase,which has a serious impact on the environment.Therefore,it is of great significance to develop clean and renewable energy.The combustion product of hydrogen is water and has a very high calorific value,which is regarded as one of the most potential alternative energy sources.However,hydrogen is flammable and explosive,and how to achieve safe and efficient storage and transportation is a key problem that needs to be solved urgently.Formic acid has a hydrogen density of 4.4 wt%,is low toxicity and is biodegradable.At the same time,it is liquid at room temperature for easy storage and transportation.Formic acid hydrogen storage was prepared by hydrogenation reaction using CO₂ as carrier.Further catalyzing the decomposition of formic acid to liberate hydrogen can realize the reversible storage and release of hydrogen with good renewability and environmental benefits.In this thesis,a series of alkylamino-or hydroxyl-containing pyridine amide ligands and their corresponding functionalized iridium complexes were designed and synthesized.The structures were characterized by nuclear magnetic resonance spectroscopy,high-resolution mass spectrometry,and single crystal X-ray diffraction.Subsequently,these functionalized complexes were applied to the study of reversible hydrogen storage and desorption with CO₂ as carrier.Catalytic CO₂ hydrogenation to formate:Catalysts were initially screened under the same conditions.Among them,the pyridine amide chloride catalyst Cat-6 with NEt₂ substituent and hydroxyl pendant has the best activity.Then the type of base,the amount of base,the amount of catalyst,reaction temperature,reaction pressure and reaction time were optimized.The reaction was carried out for 36 h under the optimal conditions of 1 mol/L KHCO₃ aqueous solution（10mL）,catalyst Cat-6 dosage of 0.05μmol,temperature of 25℃,and reaction pressure of 3.0MPa（H₂/CO₂=1:1）,TON value up to 81760.The formation of Ir-[H]intermediate was detected by ¹H NMR.The p Ka value of Cat-6 was determined by UV-Vis titration experiments,and the reaction mechanism of catalytic hydrogenation was proposed.Catalytic formic acid dehydrogenation:The effect of pH value on the activity of the solution was explored,and the results showed that the TOF value first increased and then decreased with the increase of pH value.Pyridine amide catalyst Cat-2（0.5μmol）with NMe₂substituent in 1 mol/L formic acid/sodium formate mixed solution（pH=2.45,10 mL）at 60℃,initial TOF values for the first 10 minutes up to 38441 h^-1.The reaction temperature has been optimized,and the initial TOF of Cat-2 can reach 108105 h^-1 in the first 5 minutes at 80℃.When the formic acid concentration was as high as 15 mol/L,Cat-2 still had high catalytic activity,and the initial TOF in the first ten minutes at 60℃ could reach 13043 h^-1.The reaction was carried out for 24 h,the TON value was as high as 299812.The rate-controlling steps of the reaction were explored by deuterated kinetic isotope experiments.The p Ka value of Cat-2 was determined by UV-Vis spectroscopy titration experiment.The existence of active amide N-H in the catalyst was confirmed by ¹H NMR,and the mechanism of catalytic dehydrogenation of formic acid was finally proposed.A series of functionalized iridium complexes synthesized in this thesis have good activities for CO₂ hydrogenation to formate and formic acid dehydrogenation.It provides a beneficial exploration for the development of a highly efficient reversible hydrogen storage and release system using CO₂ as the carrier in the aqueous phase.