Development And Investigation Of Iridium Complexes For Catalytic Dehydrogenation Of Formic Acid In Water

The consumption of un-renewable fossil resources increases rapidly with the population growing on earth,causing severe contamination of environment.Thus,it is one of great challenge to develop new renewable energy.Hydrogen has drawn great attention of researchers due to its properties of renewability and environmental friendliness.Storage and transportation of hydrogen in secure and efficient ways will be key to implement hydrogen energy economy and hydrogen application.Formic acid is one of the promising hydrogen storage materials owing to its properties of thermodynamically stability,non-toxic organic acid,low-volatility liquid at room temperature,and relatively high hydrogen content of 4.3 wt%.Homogeneous catalysts have advantages of high reaction rates and selectivity for the decomposition of FA to hydrogen and carbon dioxide.Among the numerous homogeneous catalysts,biomimetic catalysts inspired by the structure of active site in hydrogenase have attracted increasing attention.In this study,a series of water soluble catalysts inspired by the structure of [FeFe]-hydrogenase bearing pendent pyridine as pendent base has been developed for catalytic FA dehydrogenation.Optiaml catalytic condition and reaction mechanism were explored.(1)Several iridium complexes with pyridine N atom as pendent base were synthesized.They have advantages of easy to synthesis,structural stability,and good water-solubility.(2)Reaction conditions for catalytic FA dehydrogenation were optimized.Reaction temperature,FA concentration,and solution pH were studied.Finally,the optimal reaction conditions were obtained: reaction temperature of 60 °C,pH = 1.7,1 mol/L of FA,atmosphere pressure.Under optimized conditions,Cat.1 and Cat.6 afforded high initial TOF of 6600 h-1 and 5757 h-1,respectively.(3)Activation energy of 67.5 kJ mol-1 and 75.7 kJ mol-1 for Cat.1 and Cat.6 were calculated respectively from the Arrhenius plots.Under optimized conditions,no CO was detected in the released gas,suggesting good selectivity of the catalysts.(4)Based on computational and experimental investigation of the mechanisms,it was demonstrated that pendent pyridine of Cat.1 formed pyridine cation with a proton,which facilitates the proton transfer via 1H-pyrazole units by proton relay and provides a low energy pathway for the hydrogen generation,thus promoting its catalytic activity.The study provided novel bioinspired catalysts for FA dehydrogenation in water.The proposed mechanism based on computational and experimental investigation revealed the significant effect of pendent pyridine.This study provided important theory basis for designing new catalysts with N atom containing heterocycles as functional groups.