Hydrogen Storage Using Formic Acid Catalyzed By Iridium Complexes Bearing Hydroxyl Or Amino Group

With the development of economy and society,people's demand for energy is getting higher.Fossil energy is a well-known non-renewable energy with limited reserves on the earth.The generated CO₂ causes the greenhouse effect and a series of environmental and climate problems.Therefore,there is an urgent need for new renewable energy sources.Hydrogen energy is an ideal alternative energy.It has high calorific value and causes no pollution.It can be used in fuel cells or direct combustion.Due to the low volume density of hydrogen,the storage and transportation problems have not been solved,which restricts the application of hydrogen energy.Formic acid has many advantages such as cheap,non-toxic and bio-degradable,relatively high hydrogen content,convenient transportation,etc.It is one of the most promising hydrogen storage materials.Inspired by[FeFe]-hydrogenase and[Fe]-hydrogenase,a series of functionalized iridium complexes was designed and synthesized.They were used to catalyze the hydrogenation of CO₂ and the dehydrogenation of formic acid in this thesis.The performance of the catalysts is evaluated under the optimized reaction conditions.A catalytic mechanism was proposed according to our experimental results.For the reaction of CO₂ hydrogenation to formic acid,we screened the complexes,base dosage,reaction pressure,catalyst loading,reaction temperature and reaction time.Cat 3showed the best catalytic activity.Compared with other catalysts,Cat 3 ortho-hydroxyl as apendent base can promote hydrogen heterolysis,so catalytic activity is better than catalysts bearing OCH₃and H.For catalysts bearing amino groups on the pyridine ring,the para position is better than the ortho position;the methyl group on the pyrazole ring can enhance the electron donating capacity,and the catalytic activity is better than that the catalysts bearing CH₂OH and H,indicating that the remote hydroxyl group cannot promote hydrogen heterolysis.Under the optimal reaction conditions,100°C,catalyst loading 0.25?mol,2 MPa H₂/CO₂(1:1),the TON can reach 5432 after 36 hours.We successfully detected Ir-hydride by¹H NMR spectroscopy,and proposed a possible reaction mechanism catalyzed by Cat 3.In the catalytic formic acid dehydrogenation reaction,we determined Cat 3 as the optimal catalyst,and screened out the optimal reaction conditions as follows:catalyst loading1?mol,reaction temperature 80?,p H 3.79,1 mol/L formic acid/sodium formate(3:7)mixture solution.Under the optimized conditions,the initial TOF of Cat 3 can reach 12725h^-1.The Arrhenius plot was drawn with Cat 3 under different temperature and the activation energy of the reaction was calculated to be 41 k J mol^-1.Cat 3 exhibited relatively low activation energy and good catalytic activity.The functionalized iridium complexes bearing hydroxyl or amino group synthesized in this thesis showed good catalytic activity for the hydrogenation of CO₂ to formic acid and the dehydrogenation of formic acid.This study provided a useful exploration for the design and development of efficient catalysts for hydrogen storage using formic acid.