Heterogeneous Silica-Supported Ruthenium Phosphine Catalysts for Selective Formic Acid Decomposition

The water-soluble ruthenium(II)–mTPPTS complex (mTPPTS= meta-trisulfonated triphenylphosphine) efficiently catalyzes the generation of hydrogen from formic acid in aqueous solution. This catalyst has been immobilized in a hydrophilic silica sup- port to facilitate mobile portable applications. It was used in diluted formic acid solutions, in which the recycling of a liquid phase catalyst is problematic. A series of heterogenized catalysts have been prepared by the reaction of the ruthenium(II)– mTPPTS dimer and silica functionalized with diphenylphos- phine groups via alkyl chains. The length of the alkylene chain separating the silica from the diphenylphosphine group has been varied in these catalysts—all of them catalyzing the decomposition of formic acid. The catalysts were easily separated from the solution and reused. The optimized catalytic system based on MCM41-Si-(CH2)2PPh2/Ru-mTPPTS demonstrated an activity and stability comparable to those of the homogeneous catalyst : a turnover frequency of 2780 h-1 was obtained at 1108C, and no ruthenium leaching was detected after turnover numbers of 71000.

Kinetics and Mechanism of Formic Acid Decomposition in Aqueous Solution using Ruthenium Pre-catalysts with Hydrophilic Phosphine Ligands

Weijia Gan

Hydrogen holds the potential to be an alternative to replace fossil fuels in the future. The tremendous research effort dedicated to the issue of hydrogen storage has led to considerable advancements in the development of both adsorption materials and chemical storage. Still, for many of these systems the gap between current state of research and industrial application is considerable. When it comes to systems for the reversible storage of hydrogen, the HCOOH/CO2 cycle currently shows the most promises. Its physical-chemical properties make formic acid an ideal system for hydrogen storage. Herein, a series of homogeneous catalytic systems have been developed for hydrogen generation from formic acid in aqueous solution. These catalysts are formed from commercially available RuCl3 with a variety of water-soluble phosphine ligands. These ligands vary in the type, number and position of their hydrophilic substituents. Most of them allow the efficient decomposition of formic acid under mild conditions for a large number of catalytic cycles. Especially a new series of cationic phosphine ligands provide the catalytic systems with high activity. Due to the observed high initial reaction rate of many catalytic systems, mechanistic studies have also been undertaken with important intermediates and a catalytic cycle being proposed. Accordingly, the heterogeneous catalysts have also been developed based on the immobilization of homogeneous ruthenium complexes. The immobilized catalysts also afford hydrogen with high purity and allow easy catalyst separation, overcoming the previous limitations in the use of homogeneous catalysts, particularly for mobile/portable applications. The research undertaken in this thesis is presented in three main parts: (i) the development and optimization of homogeneous catalytic systems; (ii) the development and optimization of heterogeneous/immobilized catalytic systems; (iii) the mechanistic studies on the initial fast catalytic cycle of homogeneous catalysts.

EPFL2012

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.