Hydrogen storage in the carbon dioxide/formic acid system, using homogeneous iron(II)-phosphine catalysts in aqueous solution

The valorization of carbon dioxide and its transformation into useful chemicals is essential as mankind consumes more and more fossil fuels, thus producing equivalent wastes. The storage of hydrogen, a promising energy carrier, is also of great interest in developing a sustainable future. Here, we present our results on aqueous phase formic acid (FA) dehydrogenation reaction using non-noble metal, iron based pre-catalysts. We have synthesized the m-trisulfonated-tris[2-(diphe nylphosphino)ethyl]phosphine sodium salt (PP3TS), a water soluble polydentate ligand. The catalysts, with iron (II), were formed in situ and were active in homogeneous catalytic, selective formic acid dehydrogenation resulting in H2 and CO2 from aqueous formic acid solutions. This required no organic co-solvents, bases or any additives. Manometry, multinuclear NMR and FT-IR techniques were used to follow the dehydrogenation reactions, calculate kinetic parameters, and analyze the gas mixtures for purity. The iron (II) catalyst is entirely selective and the H2 and CO2 gas mixture is free from CO contamination. To the best of our knowledge, these represent the first examples of first row transition metal based catalysts that dehydrogenate quantitatively formic acid in aqueous solution. The reverse reaction, the direct CO2 hydrogenation using an iron (II) phosphine catalyst is also presented here. In water, using the same iron (II) catalyst precursor with PP3TS as the ligand, up to 0.5 M of formic acid can be produced without any additives, i.e. in acidic aqueous solutions. These results were obtained at room temperature and under hydrogen and carbon dioxide pressures. The system is not sensitive to oxygen or air exposure. Therefore, his carbon dioxide reduction and formic acid dehydrogenation cycle can be repeated several times without the loss of the catalyst activity. Hydrogen is regarded as one of the future energy carriers. Using this Fe(II)-PP3TS catalyst, a reversible hydrogen storage system can be realized by a battery-like charge/discharge mechanism. This allows for a clean storage of energy in the form of formic acid as well as the safe delivery of hydrogen gas to proton exchange membrane fuel cells.