A Study of Homogeneous Hydrogenation of CO2 to Formic Acid and Formic Acid Dehydrogenation

Abstract

The concentration of CO2 in the atmosphere continues to increase due to an escalating rate of fossil fuel combustion, causing ever mounting concern about climate change. Meanwhile CO2 is an abundant, safe and renewable C1 source. Thus transformation of CO2 to value-added chemicals is of great significance and has been extensively investigated in the past decades. Homogeneous hydro-genation of CO2 to formic acid (FA) especially has drawn significant attention, not only because FA itself has various applications including FA fuel cell, but also because FA can be a good hydrogen storage medium. The hydrogenation of CO2 using such as solar-generated H2 to produce FA, which is readily transport-able, and the dehydrogenation of FA to release H2 provide a decent strategy for hydrogen storage via CO2 recycling. However, it remains a challenge for the efficient hydrogenation of CO2 to FA and FA dehydrogenation under mild conditions, especially for the aqueous phase reaction. Bearing these in mind, this work focus on the development of homoge-neous catalysts for this reversible transformation and the main results are sum-marized as follows: 1. Cyclometallated [IrCp*(N^C)Cl] complexes (N^C = 2-aryl imidazoline ligand) have been synthesized as the catalyst for the homogeneous hydrogenation of CO2 to formate, and satisfactory reactivity (TOF: 57330 h-1，TON: 102200) was achieved at 80 oC in a mixture solution of MeOH/H2O. Water has been found to play an important role in this reaction and the catalysts with hydrophilic ligands show much higher activity in the hydrogenation of CO2 than those with hydro-phobic ligands. 2. Considering the electronic property of the ligand and catalyst water solubility, water-soluble iridium complexes [IrCp*(N,N’)Cl]Cl bearing conjugated N,N’-diimine ligands were subsequently synthesized and applied to CO2 hydro-genation. This catalyst is capable of efficiently hydrogenating CO2 to produce FA directly in water without any additives under mild reaction conditions. At 80 oC, the reaction turnover frequency (TOF) exceeds 13000 h-1 at 5.0 MPa of H2/CO2 (1:1). 3. FA dehydrogenation to H2 and CO2 is studied using the iridium catalyst [IrCp*(N,N’)Cl]Cl in water without any additives, affording a TOF of 487500 h-1 at 90 oC and a turnover number (TON) of 2400000 with in situ prepared catalyst at 80 oC, the highest values reported for FA dehydrogenation to date. 4. In addition, the FA dehydrogenation is shown to be catalyzed by [RhCp*Cl2]2 in the FA/NEt3 azeotrope and is found to be accelerated by the addition of simple halide ions, especially I-.