Homogeneous Catalysis for Sustainable Hydrogen Storage in Formic Acid and Alcohols

Hydrogen gas is a storable form of chemical energy that could complement intermittent renewable energy conversion. One of the main disadvantages of hydrogen gas arises from its low density, and therefore, efficient handling and storage methods are key factors that need to be addressed to realize a hydrogen-based economy. Storage systems based on liquids, in particular, formic acid and alcohols, are highly attractive hydrogen carriers as they can be made from CO2 or other renewable materials, they can be used in stationary power storage units such as hydrogen filling stations, and they can be used directly as transportation fuels. However, to bring about a paradigm change in our energy infrastructure, efficient catalytic processes that release the hydrogen from these molecules, as well as catalysts that regenerate these molecules from CO2 and hydrogen, are required. In this review, we describe the considerable progress that has been made in homogeneous catalysis for these critical reactions, namely, the hydrogenation of CO2 to formic acid and methanol and the reverse dehydrogenation reactions. The dehydrogenation of higher alcohols available from renewable feedstocks is also described. Key structural features of the catalysts are analyzed, as is the role of additives, which are required in many systems. Particular attention is paid to advances in sustainable catalytic processes, especially to additive-free processes and catalysts based on Earth-abundant metal ions. Mechanistic information is also presented, and it is hoped that this review not only provides an account of the state of the art in the field but also offers insights into how superior catalytic systems can be obtained in the future.

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

Carbon dioxide for hydrogen storage via formic acid derivatives and methanol

Katerina Stefania Sordakis

Securing our energy future while minimizing the associated environmental impacts is a challenging endeavor and the fundamental aspect of sustainable development. The choice of energy resource and the reduction of greenhouse gas emissions, primarily carbon dioxide (CO2), are key parameters in this process. Hydrogen gas (H2) is an energy carrier with the potential of "green" production and utilization, in addition to having attractive inherent fuel properties. However, complications during its storage arise from its intrinsically light nature. Chemical H2 fixation within liquid formic acid (FA) and its derivatives is therefore a promising approach. In the present dissertation, reversible H2 storage cycles based on homogeneous FA/formate dehydrogenation and CO2/bicarbonate hydrogenation, as well as the related CO2 transformation to methanol are investigated. The first chapter provides an overview of challenges underlying energy sustainability, prospects for integration of H2 with our energy infrastructure, the employment of FA as a liquid organic hydrogen carrier and state of the art homogeneous catalysts for selective FA dehydrogenation and CO2 hydrogenation. The combination of these reactions to form a closed CO2 loop is the basic concept behind a reversible H2 storage system as it is envisaged by our group. In the second chapter, FA dehydrogenation/formation in the presence of triethylamine is discussed. The results are summarized as equilibrium positions within this couple, which can be controlled by adjustment of the operating pressure and temperature. The basic additive promotes H2 fixation but also allows for on-demand H2 release from a triethylammonium formate substrate. The third chapter addresses H2 storage in basic media, i.e. aqueous bicarbonate hydrogenation, with a Ru(III) catalyst precursor and a number of water-soluble phosphine ligands. The latter are introduced to tailor the exhibited activity by affecting the steric and electronic parameters of the catalyst. All phosphines provide high bicarbonate conversions, albeit with low reaction rates. Chapter four summarizes the optimization and mechanistic studies of the reversible formate dehydrogenation reaction, in the presence of a Ru(II)-mTPPTS catalyst. Cesium is chosen as the cation for the formate/bicarbonate salts because it leads to their high solubility in the aqueous solvent and therefore an increase in the hydrogen storage capacity. The involved equilibria are determined and a reaction mechanism for the formate dehydrogenation step is proposed, based on NMR studies. Advantages of the presented approach include the utilization of water as the solvent, the absence of CO2 throughout the reactions and the successful in situ recycling of the environmentally benign hydrogen carrier. In Chapter five the feasibility of producing both FA and methanol directly from CO2, at room temperature, in a "one-pot" reaction using aqueous solvent is demonstrated. Formic acid formation occurs in water without additives or organic solvents with a homogeneous iridium catalyst. Formic acid also undergoes disproportionation into methanol, with the same complex. Under optimized conditions, FA conversions of 98% and methanol selectivities of 96% are achieved in the disproportionation reaction. These reactions are relevant to the field of sustainable H2 storage, but might also provide an alternative approach to the commercial fossil-fuel-dependent production of formic acid and methanol.

EPFL2016

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