Carbon dioxide hydrogenation catalyzed by a ruthenium dihydride: a DFT and high-pressure spectroscopic investigation

Reaction pathways during CO2 hydrogenation catalyzed by the Ru dihydride complex [Ru(dmpe)2H2] (dmpe=Me2PCH2CH2PMe2) have been studied by DFT calculations and by IR and NMR spectroscopy up to 120 bar in toluene at 300 K. CO2 and formic acid readily inserted into or reacted with the complex to form formates. Two formate complexes, cis-[Ru(dmpe)2(OCHO)2] and trans-[Ru(dmpe)2H(OCHO)], were formed at low CO2 pressure (<5 bar). The latter occurred exclusively when formic acid reacted with the complex. A RuH⋅⋅⋅ HOCHO dihydrogen-bonded complex of the trans form was identified at H2 partial pressure higher than about 50 bar. The trans form of the complex is suggested to play a pivotal role in the reaction pathway. Potential-energy profiles along possible reaction paths have been investigated by static DFT calculations, and lower activation-energy profiles via the trans route were confirmed. The H2 insertion has been identified as the rate-limiting step of the overall reaction. The high energy of the transition state for H2 insertion is attributed to the elongated Ru−O bond. The H2 insertion and the subsequent formation of formic acid proceed via Ru(η2-H2)-like complexes, in which apparently formate ion and Ru+ or Ru(η2-H2)+ interact. The bond properties of involved Ru complexes were characterized by natural bond orbital analysis, and the highly ionic characters of various complexes and transition states are shown. The stability of the formate ion near the Ru center likely plays a decisive role for catalytic activity. Removal of formic acid from the dihydrogen-bonded complex (RuH⋅⋅⋅HOCHO) seems to be crucial for catalytic efficiency, since formic acid can easily react with the complex to regenerate the original formate complex. Important aspects for the design of highly active catalytic systems are discussed.

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

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

Catalytic hydrogenation of alkenes under biphasic conditions

Corinne Daguenet

Ruthenium(II)-arene diphosphine complexes are shown to be effective precursors for the catalytic hydrogenation of the C=C bond in an aqueous biphase using molecular hydrogen as the hydrogen source. A mechanistic study performed in water, utilizing high gas pressure NMR spectroscopy and electrospray ionization mass spectrometry, and experiments involving the variation of pH, enables indirect identification of the catalytically active species resulting from the substitution of the chloride with the dihydrogen ligand. Similar catalytic experiments are carried out in ionic liquids (ILs), such as [bmim][BF4] and [bmim][TfO] using an analogous precursor, but no conversion is observed and the ruthenium(II) species does not undergo any chemical change during the reaction. The dissociation of chloride, leading to the catalyst, is inhibited in these ILs due to a lower solvation of chloride. An estimate of the bmim-chloride interaction, which is lower than the water-chloride interaction, is determined by 1H NMR spectroscopy. To ensure the formation of the active species in such ILs, the coordinated chloride is abstracted beforehand and replaced by acetonitrile, which restores the catalytic activity. An alternative approach to overcome the chloride dissociation problem consists of improving the solvation of the chloride in the IL medium. To accomplish this, water is added as a co-solvent in the IL phase and, under these conditions, the activity is restored. In order to assess the reactivity of chloride ions in ILs, a relative scale for the solvation of chloride is given for a series of [cation][Tf2N] ILs. This scale, based on the reaction enthalpy for the dissociation of a chloride-templated metallacage, yields the following order of chloride solvation: [C5e3am][Tf2N] < [bmpy][Tf2N] < [bmim][Tf2N] ≤ [bdmim][Tf2N] < [bpy] [Tf2N] ≪ [mimeOH][Tf2N]. In Tf2N-based ILs, the catalytic activity of hydrogenation reactions using the same Ru(II) precursor in the presence of chloride is totally modified. In a series of [cation][Tf2N] ILs, the rate of formation of the catalyst, which is promoted by chlorides, is shown to depend on the solvation of chloride.

EPFL2006