Catalyst design for C-O bond hydrogenolysis of renewable materials.

Anthropogenic carbon dioxide emissions leading to climate change require to use of renewable carbon sources such as CO2 and biomass which differ from fossil resources by having a higher number of oxygen atoms. Therefore, catalytic C-O bond cleavage will play a pivotal role in their conversion into carbon neutral fuels, materials and chemicals. This thesis will focus on the most challenging substrate for selective C-O bond hydrogenolysis, diaryl ether present in lignin (i.e. one of the components of biomass), summarise the state of research and improve the comprehension in the characteristics a catalyst requires to selectively cleave these bonds without altering other functionalities. We showed that the modification in the electronic state and the resulting polarity between two different metals present in a bimetallic nanoparticle favour the selectivity towards hydrogenolysis of a polar C-O bonds over aromatic ring hydrogenation. In addition, we demon-strate that single metal sites cannot hydrogenate an aromatic ring and hence are selective. Final-ly, we applied our knowledge in C-O bond cleavage in CO2 conversion using propylene carbonate as a relay molecule to produce propylene glycol and methane.

Development of Catalytic Umpolung

Jonathan Brand

The development of efficient synthetic methodologies is crucial to access complex molecules in an economic, eco-friendly and safe way. The reversal of the intrinsic reactivity of functional groups, called Umpolung, allows alternative synthetic strategies and can improve synthetic efficiency. In this thesis, two new Umpolung methods have been investigated. First a novel approach for the direct alkynylation of aromatic C-H bonds has been developed to access aromatic alkynes. Secondly, novel bifunctional (thio)urea carbene catalysts have been synthesized and their use in asymmetric intermolecular heterocouplings has been investigated. Aromatic alkynes are compounds of utmost prominence as synthetic intermediates and for application in materials science. Classical synthetic methods require a prefunctionalization of the aromatic ring in addition to the alkynylation step. An alternative emerging strategy to introduce triple bonds is based on the Umpolung of acetylenes by the use of electrophilic acetylene reagents. In addition, the use of metal catalysis has recently arisen to couple aromatic C-H bonds with electrophiles in a single step. However, direct alkynylation has been only scarcely investigated. In this thesis, alkynyl hypervalent iodines, highly electrophilic acetylenes, were used for the metal-catalyzed direct alkynylation of aromatic C-H bonds. Indoles were alkynylated in C3 position using AuCl as catalyst and TIPS-EBX (1- [(triisopropylsilyl)ethynyl]-1λ3,2-benziodoxol-3(1H)-one) as source of electrophilic acetylene. The reaction proceeded under mild open-flask conditions (room temperature under air, commercial solvents). Importantly, it was the first example showcasing the superiority of alkynyl benziodoxolones over alkynyl iodoniums in alkyne transfer reactions. On contrary to other alkynylation methods, which have either a limited scope or require indole protection, this reaction tolerated a large range of functional groups and worked for unprotected indoles. Furthermore in the presence of pyridine, pyrroles were alkynylated in C2 position in high yields. This development was important in light of the instability of unprotected halogeno- pyrroles, which makes their use in cross-coupling reactions difficult. A one-pot 2- alkynylaniline cyclization/direct alkynylation was next developed. The influence of substituents on the acetylene, the iodoxole ring structure and the benzene ring substitution on the alkynyl benziodoxolones were examined. This study revealed that bulky silyl groups were ideal alkyne substituents. Preliminary mechanistic investigations indicated that the reaction proceeded either via a π activation mechanism or via an oxidative AuI/AuIII mechanism. iii Unfortunately it has not yet been possible to distinguish between both mechanisms. The alkynylation of electron-rich arenes was next explored. N,N-dialkylanilines were alkynylated with high para regioselectivity using iPrOH as solvent. This transformation represented the first one-step synthesis of para-alkynylanilines and was then extended to 1,3,5- trimethoxybenzenes.

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

CO Methanation for Synthetic Natural Gas Production

Tilman J. Schildhauer

Energy from woody biomass could supplement renewable energy production towards the replacement of fossil fuels. A multi-stage process involving gasification of wood and then catalytic transformation of the producer gas to synthetic natural gas (SNG) represents progress in this direction. SNG can be transported and distributed through the existing pipeline grid, which is advantageous from an economical point of view. Therefore, CO methanation is attracting a great deal of attention and much research effort is focusing on the understanding of the process steps and its further development. This short review summarizes recent efforts at Paul Scherrer Institute on the understanding of the reaction mechanism, the catalyst deactivation, and the development of catalytic materials with benign properties for CO nnethanation.