Synthesis And Characterization Of Homogeneous Iridium Catalyst For Catalytic Dehydrogenation Of Formic Acid

Hydrogen is an excellent energy storage carrier to solve the problem of portable storage of green renewable energy,but its low energy volume density limits its application in the field of portable energy storage.Formic acid is an ideal liquid hydrogen storage carrier,which is environmentally friendly and has high hydrogen storage density.The catalytic dehydrogenation of formic acid can be realized by homogeneous transition metal complexes with high activity and selectivity under relatively mild conditions.The half-sandwich type catalysts synthesized by bipyridine and amine N,N-bidentate ligands have been proved to successfully applied in the dehydrogenation of formic acid,but the research on the application of monopyridine and diimine ligands is less widely investigated in the field.Therefore,we designed a series formic acid dehydrogenation catalyst,and achieved progresses are as follows:（1）A series of mono-pyridine ligands（A1～A4）and conjugate amines N,N-bidentate ligands（B1～B4）were designed and coordinated with[Cp^*Ir（H₂O）₃]SO₄,and TOFs ranging from 729.2 to 8593.8 h^-1（Turnover frequency）were achieved in preliminary screening experiments.（2）Based on the preliminary screening test data,the most promising catalyst with N,N-bidentate conjugated amine B4 was selected for the conditional optimization experiment.Finally,the TOF of 46875.0 h^-1 was achieved at the reaction temperature of 90℃,the concentration of formic acid was 3.6 mol/L（Fomic acid:Sodium formate=19:1）and the amount of catalyst was 1μmol/L.（3）B4 catalyst can catalyze dehydrogenation of formic acid with high activity in 0.45～13.5 mol/L formic acid solution,and finally achieve a TON（Turnover number）of more than 75208 in 10.8 mol/L formic acid solution.The activation energy of dehydrogenation of formic acid catalyzed by B4 catalyst is 55.6 k J·mol^-1 through drawing Arrhenius curve.（4）The effect of high temperature on the activity of B4 catalyst was studied by pretreatment at high temperature.Based on the experimental results of oxidative pretreatment,it was confirmed that B4 catalyst had antioxidant properties,could catalyze formic acid dehydrogenation in aerobic environment,and could be stored for a long time in aerobic low-temperature environment.Finally,based on KIE experiment and conditional optimization experiment results,the reaction mechanism of formic acid dehydrogenation catalyzed by B4 was proposed,β-H elimination involving the capture of C-H bond in HCOOH was the rate-determining step.