Selective Formic Acid Decomposition for High-Pressure Hydrogen Generation: A Mechanistic Study

A homogenous catalytic system has been developed that efficiently and selectively decomposes formic acid into hydrogen and carbon dioxide. [Ru(H2O)6]2+, [Ru(H2O)6]3+ and RuCl3·xH2O are all excellent precatalysts in presence of TPPTS (TPPTS=meta-trisulfonated triphenylphosphine), the formic acid decomposition taking place in the aqueous phase, under mild conditions and over a large range of pressures. Optimisation of the reaction conditions is described together with a detailed mechanistic study leading to a tentative catalytic cycle. The performance of the catalytic system for continuous hydrogen generation is presented. Overall, the method proposed overcomes the limitations of other catalysts for the decomposition of formic acid making it a viable hydrogen-storage material.

Direct Carbon Dioxide Hydrogenation into Formic Acid in Acidic Media

Séverine Joëlle Moret

A major challenge in the face of increasing global energy demand is the development of alternative environmentally friendly and renewable energy resources. Hydrogen is an excellent energy carrier, but has drawbacks in storage density and is dangerous to handle. An alternative approach involves controlled release of hydrogen from suitable materials, and an excellent candidate for this is formic acid. Because hydrogen production from formic acid is more convenient in acidic aqueous solutions, development of direct carbon dioxide hydrogenation has to be carried out under the same acidic conditions that provides a direct reversible hydrogen storage system. As current sources of formic acid are derived from fossil fuels, emphasis must be placed on conversion of renewable carbon sources to this materials. This thesis examines the direct hydrogenation of carbon dioxide to formic acid using homogeneous catalysts. Importantly, this reaction was achieved in base and additive-free conditions. Direct formic acid synthesis, without any additives has several advantages, including simpler isolation of the product. In addition, formic acid can also be used as a feedstock chemical and thus contributing to decrease the released CO2 to the atmosphere and consequently to the possible reduction of the greenhouse effect. A homogeneous catalytic system based on a [RuCl2(PTA)4] pre-catalyst active for carbon dioxide hydrogenation without the need for additives was developed. While unprecedented conversions were achieved in aqueous mixtures, a remarkable solvent effect was discovered for dimethyl sulfoxide. Furthermore, both catalytic systems are air stable and highly robust allowing for multiple recycling runs. Studies on the activity of the ruthenium pre-catalyst in the reverse reaction of hydrogen delivery in dimethyl sulfoxide have been undertaken and revealed promising results to achieve a CO2 neutral catalytic cycles. Mechanistic studies were carried out using NMR spectroscopy and provide valuable insights into the reasons for the high activities for the catalyst. After a general introduction on current hydrogen storage technologies, the research undertaken in this thesis will be presented in three main parts: (1) Optimization of the high pressure sapphire NMR techniques used for the study of systems under pressure; (2) Development of homogeneous ruthenium catalytic systems for the direct hydrogenation of carbon dioxide to formic acid; (3) Mechanistic studies on the direct carbon dioxide hydrogenation using the [RuCl2(PTA)4] pre-catalyst

EPFL2014

Homogeneous catalytic decomposition of formic acid for high pressure hydrogen generation

Céline Fellay

The main obstacles in the widespread use of hydrogen as energetic alternative to fossil fuels are storage problems intrinsically linked to its extremely light and explosive nature. Since no ideal storage systems currently exist, reversible fixation of hydrogen is a very active field of research. Formic acid has previously been proposed as hydrogen storage material using heterogeneous catalysts to decompose it. These systems, however, lose activity over time and/or have low selectivity. Formic acid decomposition has been revisited during the course of this thesis in attempt to provide a catalytic system that efficiently and selectively decomposes formic acid into H2 and CO2. Accordingly, a homogeneous catalytic system to decompose formic acid in aqueous phase has been developed. The catalytically active species are formed in situ from [Ru(H2O)6]2+ or RuCl3, and sulfonated phosphine ligands. The decomposition takes place under mild conditions, over a large range of temperatures (25-170°C) and pressures (1-750 bar, at least). The robustness of the catalytic systems allows a very large number of catalytic runs to be performed without any deactivation, hence, continuous hydrogen production could be achieved. Mechanistic investigations allowed the characterisation of several intermediates and a catalytic cycle has been proposed. The selective catalytic process developed affords hydrogen essentially free of CO, at desired rates and pressures, overcoming previous limitations in the use of formic acid as a hydrogen storage material. The research undertaken in this thesis is presented in three parts: (i) development and optimisation of the catalytic system in batch mode, (ii) mechanistic investigations on the optimised catalysts, and (iii) development of a continuous hydrogen production system.

EPFL2008

Hydrogen Storage and Delivery: The Carbon Dioxide – Formic Acid Couple

Gabor Laurenczy

Carbon dioxide and the carbonates, the available natural C-1 sources, can be easily hydrogenated into formic acid and formates in water; the rate of this reduction strongly depends on the pH of the solution. This reaction is catalysed by ruthenium(II) pre-catalyst complexes with a large variety of water-soluble phosphine ligands; high conversions and turnover numbers have been realised. Although ruthenium(II) is predominant in these reactions, the iron(II) - tris[(2-diphenylphosphino)-ethyl]phosPhine (PP3) complex is also active, showing a new perspective to use abundant and inexpensive iron-based compounds in the CO2 reduction. In the catalytic hydrogenation cycles the in situ formed metal hydride complexes play a key role, their structures with several other intermediates have been proven by multinuclear NMR spectroscopy. In the other hand safe and convenient hydrogen storage and supply is the fundamental question for the further development of the hydrogen economy; and carbon dioxide has been recognised to be a viable H-2 vector. Formic acid - containing 4.4 weight % of H-2, that is 53 g hydrogen per litre - is suitable for H-2 storage; we have shown that in aqueous solutions it can be selectively decomposed into CO-free (CO < 10 ppm) CO2 and H-2. The reaction takes place under mild experimental conditions and it is able to generate high pressure H-2 (up to 600 bar). The cleavage of HCOOH is catalysed by several hydrophilic Ru(II) phosphine complexes (meta-trisulfonated triphenylphosphine, mTPPTS, being the most efficient one), either in homogeneous systems or as immobilised catalysts. We have also shown that the iron(II) - hydrido tris[(2-diphenylphosphino)ethyl]phosphine complex catalyses with an exceptionally high rate and efficiency (turnover frequency, TOF = 9425 h(-1) mol(-1); turnover number, TON = 92400) the formic acid cleavage, in environmentally friendly propylene carbonate solution, opening the way to use cheap, non-noble metal based catalysts for this reaction, too.