Base-Free Non-Noble-Metal-Catalyzed Hydrogen Generation from Formic Acid: Scope and Mechanistic Insights

The iron-catalyzed dehydrogenation of formic acid has been studied both experimentally and mechanistically. The most active catalysts were generated in situ from cationic Fe-II/Fe-III precursors and tris[2-(diphenylphosphino)ethyl]phosphine (1, PP3). In contrast to most known noble-metal catalysts used for this transformation, no additional base was necessary. The activity of the iron catalyst depended highly on the solvent used, the presence of halide ions, the water content, and the ligand-to-metal ratio. The optimal catalytic performance was achieved by using [FeH(PP3)]BF4/PP3 in propylene carbonate in the presence of traces of water. With the exception of fluoride, the presence of halide ions in solution inhibited the catalytic activity. IR, Raman, UV/Vis, and EXAFS/XANES analyses gave detailed insights into the mechanism of hydrogen generation from formic acid at low temperature, supported by DFT calculations. In situ transmission FTIR measurements revealed the formation of an active iron formate species by the band observed at 1543cm(-1), which could be correlated with the evolution of gas. This active species was deactivated in the presence of chloride ions due to the formation of a chloro species (UV/Vis, Raman, IR, and XAS). In addition, XAS measurements demonstrated the importance of the solvent for the coordination of the PP3 ligand.

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

Mickael Montandon-Clerc

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.

EPFL2018

High-pressure NMR spectroscopic and calorimetric studies on formic acid dehydrogenation and carbon dioxide hydrogenation

Cornel Fink

Our current energy production is convenient and simple, but it is not sustainable. The negative impacts on our environment are omnipresent in the form of smog, extinction of species, and global warming. A major challenge of our generation is the transition to new, responsible methods of energy production. The maxim is renewable energy. Hydrogen could serve as an intermediary energy carrier. However, problems arise with the storage of hydrogen due to safety concerns and low volumetric energy density. The reduction of carbon dioxide (CO2) to formic acid (FA) is an elegant process to âcompressâ hydrogen by transforming it into a chemical which is easier and safer to handle. In this dissertation, catalytic FA dehydrogenation, CO2 reduction and calorimetric studies of the involved thermodynamics are described. The first chapter provides a global overview, beginning with general considerations about energy economy, then crossing over to hydrogen storage and infrastructure for a future hydrogen economy, before discussing FA as a hydrogen storage medium and summarizing the developments of FA dehydrogenation and CO2 hydrogenation. Chapter two describes different compounds we have studied as potential catalysts for FA dehydrogenation and CO2 hydrogenation. The focus is on complexes of Ir, Rh, and Ru regarding metals, and on Cp* and an ethylene-spaced diamine as ligands. Other motifs were also explored including Ru-phosphine complexes which afforded unexpected results. The most promising candidates are examined more closely. The third chapter summarizes our findings of formic acid dehydrogenation with homogeneous catalysts, presented in three parts. Each subchapter highlights a different structural feature or metal center. The first one focuses on Ir and Rh metal centers, equipped with Cp* and diamines. In the second section, we explore the catalytic performance of a RAPTA-type precatalyst, and the last one addresses our achievements with a rhodium catalyst containing Cp* and di(1-pyrazolyl)methane ligands. The catalysts were evaluated towards core properties such as stability (TON), activity (TOF) and recycling. Chapter four elucidates the interactions of FA with different solvents and additives, which influence the kinetics and thermodynamics of FA dehydrogenation and CO2 hydrogenation. NMR and FT-IR spectroscopy, conductometry, calorimetric measurements and computational methods were used to analyze the systems thoroughly. In chapter five, high-pressure calorimetric measurements are employed to quantify the actual energy balance during the endothermic process of FA dehydrogenation. For this purpose, we modified an off-the-shelf high-pressure reactor to suit our needs and constructed a thermo-controlled environment. Chapter six describes our research into catalytic CO2 hydrogenation to FA with two ruthenium-based phosphine pre-catalysts. One has three 1,4,7-triaza-9-phosphatricyclotridecane (CAP) ligands coordinated, which is a just recently described phosphine compound. The other complex is prepared via a straightforward synthesis route with an inexpensive, simple ligand. Both are stable in aqueous FA over weeks, making them potential catalysts for large-scale application.

EPFL2018

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

Mickael Montandon-Clerc

EPFL2018

High-pressure NMR spectroscopic and calorimetric studies on formic acid dehydrogenation and carbon dioxide hydrogenation

Cornel Fink

EPFL2018