Design, Synthesis and Characterization of New Nanoparticle Stabilizers and Their Application in Catalysis

In recent years, a strong emphasis in research towards creating sustainable and environmental friendly chemical processes has propelled the development of novel types of catalytic systems that combine advantages from homogeneous and heterogeneous processes. In this respect, transition metal based nanoparticles, TM-NPs, are excellent candidates with properties that offer high efficiency, high selectivity and the possibility of recycling. However, due to the particularly small size, within the nanoscale, TM-NPs are generally only kinetically stable and must be supported or stabilized by molecular systems to prevent aggregation into larger, less active materials. The work presented herein concerns the design, synthesis, characterisation and the applications of new TM-NP protecting agents, which are based on polymers functionalized with components that compose ionic liquids, P-ILs. Specifically, these P-ILs feature a hydrophobic poly-norbornene framework prepared using Ring Opening Metathesis Polymerization, ROMP, with pendant imidazolium-based ionic liquid substituents that allow fine tuning of the polymer solubility by anion-exchange. In Chapter 2, an aqueous, soluble P-IL containing the 1,2-dimethylimidazolium methane-sulphonate substituent and its use in the synthesis of aqueous gold NP, GNP, solutions is described. These particular TM-NPs were characterized by UV-VIS spectroscopy and Transmission Electron Microscopy, TEM. Subsequently, the P-IL/GNP solutions were used to catalyse the reduction of p-nitrophenol and the hydrogenation of cinnamaldehyde, both reactions are found to exhibit excellent activity under mild conditions. In Chapter 3, the potential of aqueous P-IL stabilizers to transfer GNPs from water to hydrophobic ILs and organic solvents is demonstrated. Using an anion-exchange process, the phase change does not result in GNP aggregation, as confirmed by UV-VIS spectroscopy and TEM analysis. Moreover, the same P-ILs were used as a protecting agents for the synthesis of aqueous rhodium NPs, RhNPs, which have been applied in the catalytic hydrogenation of natural compounds, including limonene, cinnamaldehyde, citral and α- and β-ionones. Through anion-exchange, the aqueous P-IL stabilized RhNPs are readily transferred into hydrophobic ILs, then used as catalysts in the hydrogenation of styrene and limonene. The stabilized RhNPs exhibited good activity, even after recycling. X-ray Photoelectron Spectroscopy, XPS, analysis demonstrated conclusively that the anion exchange and phase transfer of the Rh/Au-NP-IL system. In Chapter 4, three P-ILs containing poly-oxy linker groups, strategically placed between the poly-norbornene framework and the ionic groups, are described. The linkers provide additional stabilization through steric stabilization and coordination via the ether functional groups. These P-ILs were used to stabilize aqueous GNPs, which were characterized by UV-VIS spectroscopy and TEM. Their application in the reduction of p-nitrophenol reduction was demonstrated. Preliminary studies on the stabilization mode of PILs shown that the polymers, despite the presence of ionic groups, can stabilize electrostatically stabilized aqueous GNPs which otherwise aggregate in presence of water soluble imidazolium-based ILs. Moreover, they display good efficiency in terms of steric stabilization, comparable to systems featuring the poly-vinylpyrrolidone, PVP.

Amorphous Molybdenum Sulfide Films as Hydrogen Evolution Catalysts in Water

Daniel Andreas Merki

Molecular hydrogen is a promising candidate to replace fossil fuels as the energy carrier. Hydrogen does not exist in its molecular form on earth and must therefore be generated, starting from hydrogen-rich compounds. Water would be a renewable resource for hydrogen. It can be split into oxygen and hydrogen. Water splitting, however, needs electrical energy. If this energy is produced from renewable resources, such as solar or wind power, water splitting would be a sustainable and green process. One key step in water splitting is the reduction of protons to form hydrogen. To achieve a high efficiency in this reaction, catalysts are required. This dissertation is devoted to the development of hydrogen evolution catalysts that are made of earth-abundant and inexpensive materials. First, transition metal complexes of tetrathiotungstate and tetrathiomolybdate were studied as potential homogeneous catalysts for hydrogen evolution in organic or aqueous solutions (chapter 1). The redox activity of these complexes was investigated by cyclic voltammetry in the absence and presence of an acid in organic solutions. The first reduction peak of (Pr4N)2[Ni(MoS4)2] became more intense and was shifted to less negative potentials upon addition of an acid, e.g. p-cyanoanilinium tetrafluoroborate. This was considered as a sign for catalytic proton reduction. Electrolysis of a solution containing (Pr4N)2[Ni(MoS4)2] and an acid produced hydrogen. However, the complex turned out to be a precursor of the real catalytically active species, which was deposited on the working electrode’s surface during the electrochemical experiment. Consecutive cyclic voltammetric scans were found to be a good method to deposit the catalytically active film in a controllable manner from an aqueous solution of (Pr4N)2[Ni(MoS4)2]. Furthermore, a catalytically active film could be deposited from a solution of MoS42-, i.e. in the absence of Ni2+. Chapter 2 describes the investigation and characterization of films made using MoS42- as the precursor. The films consist of amorphous molybdenum sulfide. They are efficient hydrogen evolution catalysts in water. The films were characterized by XPS and electron microscopy. Whereas the pre-catalysts could be MoS3 or MoS2, the active form of the catalysts was identified as amorphous MoSx, with x close to 2. Significant geometric current densities were achieved at low overpotentials (e.g., ca. 15 mA/cm2 at η = 200 mV) using these catalysts. The catalysis was compatible with a wide range of pH values. The current efficiency for hydrogen production was quantitative. A Tafel slope of 39 mV/dec was observed, suggesting a rate-determining Heyrovsky step. The turnover frequency per active site was estimated.

EPFL2012

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

Nanoparticle - Ionic Liquid Based Catalytic Systems

Alena Karakulina

In recent years ionic liquids have emerged as an important class of compounds for the synthesis of metal nanoparticles. The significant advantage of using ionic liquids is their dual role of reaction solvent and nanoparticle stabilizer. The variability of ionic liquids enables their chemical properties to be tuned by the rational introduction of functional moieties. Metal nanoparticles and ionic liquids appear to be ideal complementary components for the construction of multifunctional catalytic systems. The development of multifunctional systems based on the combination of catalytically active nanoparticles and functionalized ionic liquids allows the integration of complex multi-step catalytic reactions in a one-pot process. The design of efficient metal nanoparticle ¿ functionalized ionic liquid catalysts is currently attracting much attention. The thesis is centered on the design, synthesis, characterisation and the catalytic applications of novel metal nanoparticle ¿ ionic liquid based catalytic systems. In the first chapter a general introduction to the field of metal nanoparticles immobilized in ionic liquids is presented, and the recent evolution of metal nanoparticle ¿ acidic ionic liquid multifunctional catalysts is reviewed. The second chapter covers the studies on chlorometallate ionic liquids. Lewis acidity of various chlorometallate systems was evaluated with respect to the speciation present in these systems. The speciation in chlorozincate ionic liquids was determined by different techniques. The obtained scale of Lewis acidity of the chlorometallate ionic liquids and the understanding of the speciation in these systems allowed accurate selection of the appropriate system for catalytic applications. The third and the fourth chapters describe the studies concerning the ability of the chlorometallate ionic liquids to function in a cooperative manner with rhodium nanoparticles in hydrogenation reactions. A novel bifunctional catalyst, consisting of rhodium nanoparticles dispersed in a Lewis acidic ionic liquid medium, was developed. In the third chapter the rhodium nanoparticle component of the catalyst is characterized. Subsequently, the combined bifunctional catalytic system was used to catalyse the hydrogenation of aromatic compounds and was found to exhibit excellent activity under mild conditions. In the fourth chapter, the high activity of the rhodium nanoparticle ¿ Lewis acidic ionic liquid catalyst the direct hydrogenation of challenging heteroarene substrates is demonstrated. Chlorozincate(II), chloroaluminate(III) and chlorogallate(III) ionic liquids were found to be effective as a second active component of the catalytic system. The chemoselective reduction of different quinolines, pyridines, benzofurans was performed at mild conditions ¿ 30 bar and 80-120°C and the corresponding heterocycles were obtained in high yields. The high selectivity of the catalyst and its tolerance to different functional groups was demonstrated on a broad range of substrates. The recyclability of the rhodium nanoparticle ¿ Lewis acidic ionic liquid catalyst was evaluated. The final chapter presents two reduced graphene oxide supported rhodium nanoparticle catalysts obtained in ionic liquid media via microwave-assisted and conventional thermal heating methods. The rhodium nanoparticle-reduced graphene oxide composites were characterized by different techniques. The catalytic performance of the rhodium nanoparticle-reduced graphene oxide composites was evaluated in the hydrogenation of quinoline compounds under mild conditions, i.e. 10 bar and 80°C. The advantages of the microwave-assisted ionic liquid synthesis for the production of highly efficient catalysts were demonstrated. The long-term stability of the catalyst obtained by microwave-assisted method was studied and the catalyst could be recycled several times without significant loss in activity and selectivity.

EPFL2016

Carbon dioxide for hydrogen storage via formic acid derivatives and methanol

Katerina Stefania Sordakis

EPFL2016

Amorphous Molybdenum Sulfide Films as Hydrogen Evolution Catalysts in Water

Daniel Andreas Merki

EPFL2012

Nanoparticle - Ionic Liquid Based Catalytic Systems

Alena Karakulina

EPFL2016