Advances in heterogeneous catalysis for the synthesis of polyoxymethylene dimethyl ethers

Our society is engaged in a major shift towards more sustainable industries. The required transition from fossils to renewable resources with low greenhouse gas emission is specifically challenging in the transportation sector where high thermal efficiency and energy density are key requirements. Due to their appealing properties, polyoxymethylene dimethyl ethers (OME) have recently received increasing attention as a new type of Diesel additive or substitute. However, the OME technology being only at an early stage of research, there is a limited understanding of the catalyst structure-activity relationships for OME synthesis. The aim of this thesis is to progress in the development of knowledge in heterogeneous catalysis for the sustainable production of OME. We focused on rational catalysts synthesis and in-depth characterization to understand how their properties influenced OME synthesis. Zeolites have demonstrated a high catalytic potential for the synthesis of OME. By modifying accessibility to the active sites in an H-ZSM-5 zeolite, we demonstrated that the reaction suffered from severe internal diffusion limitations of the reactants and products in the zeolite micropores. Controlled introduction of an intercrystalline network of mesopores significantly enhanced the zeolite's activity. Subsequently, we introduced tin in montmorillonite clay. This resulted in a hierarchical material composed of tin oxide inserted between the clay layers, with Brønsted and Lewis acidity. Its advantageous textural and acidic properties resulted in an active catalyst for OME synthesis from trioxane (TRI) and dimethoxymethane (OME1). To investigate the role of Lewis and Brønsted acidity for this reaction, a series of Beta zeolites with varying amounts of Brønsted and Lewis acid sites were synthesized. A synergy between these two types of acid sites resulted in a large turnover frequency increase accompanied with a reduction in by-product formation. Then, we studied the causes of inhibition of the reaction kinetics by water for the synthesis of OME from TRI and OME1 over an H-Beta zeolite. The presence of water as an impurity severely affected the reaction kinetics. The main OME growth mechanism shifted from direct TRI insertion to formaldehyde incorporation in OME, as the level of water in OME1 increased. The cause of deactivation was the hampered adsorption of TRI on the zeolite active sites by the presence of water. Lastly, the water free, non-oxidative catalytic dehydrogenation of methanol to formaldehyde (FA) was investigated as a source of oxymethylene groups, and thus an appealing alternative to TRI utilization. Grafting on amorphous silica in methanol was selected as the method of choice to study the activity of alkali metals. The resulting catalysts displayed an increased activity for the catalytic dehydrogenation of methanol. A large gap in selectivity towards FA between ions with a high and low charge density was observed. Na grafted on silica yielded the best combination of moderate conversion and high selectivity. As a result of the work presented in this thesis, important catalyst features can now be considered when developing catalysts or designing production processes for OME synthesis. More energy-efficient processes will require alternatives to the usage of TRI as the oxymethylene source. Alkali metals grafting on silica has the potential to catalyze this reaction and this method could pave the way for future research.

Coupling of electrocatalysts to photoabsorbers for solar fuels production

Carlos Gilberto Morales Guio

The direct transformation of sunlight into fuels and chemicals has attracted considerable attention for the potential impact on the storage of solar energies at large scales and the reduction of CO2 emissions. More than 80% of our current global energy consumption comes from the burning of non-renewable fossil fuels which produces enormous quantities of CO2 and contributes to global warming. We believe today that the key to reduce our dependence on exhaustible natural resources and limit the emission of greenhouse gases is the transition to CO2-free renewable energy sources. However, harvesting of energy from renewable sources (i.e. solar, wind, tidal) is intermittent and the deployment of devices for their transformation into useful forms of energy (i.e. electricity, chemicals, fuels, biomass) in a global scale is conditioned upon the improvement in efficiency of the energy storage mechanisms. A promising approach to store energy is the use of sunlight to drive the production of hydrogen fuel from water. Hydrogen produced from water and sunlight can be transformed back to electricity on demand or be used to generate mechanical energy in cars to power the transportation industry. The oxidation of hydrogen generates useable energy and at the same time exhaust clean, carbon-free water as the only product. Therefore, the production of hydrogen using a renewable energy source in an efficient and inexpensive device has the potential to discourage the continued use of fossil fuels and improve our environment. The viability of any mechanism for the storage of sunlight as fuel will depend on the reduction of energy penalties during the transformation process. Electrocatalysts are advanced materials that bring into one active site electrons and molecules in the appropriate orientation to lower activation energy barriers, accelerate rate of transformations and improve overall efficiencies for chemical reactions. The best catalysts for water splitting are precious metals such as Pt, Ir and Ru, which show superior rates of reaction at low overpotentials. However, these noble metals are too expensive and scarce for large scale applications. This thesis summarizes our recent research in the preparation, characterization, and application of Earth-abundant electrocatalysts for solar water splitting. Although in the last decade a great number of efficient and inexpensive electrocatalysts have been reported, methods for the deposition of these materials on the surface of photoabsorbers are rarely reported. This work shows various simple and scalable techniques for the deposition of hydrogen and oxygen evolving electrocatalysts on the surface of photocathodes and photoanodes, respectively. Surface-protected photocahodes were activated for solar hydrogen production in alkaline conditions with MoS2+x, Ni-Mo and Mo2C. A novel method for the deposition of an optically transparent amorphous iron nickel oxide oxygen evolution electrocatalyst is also reported. The catalyst was deposited on both thin film and high-aspect ratio nanostructured hematite photoanodes. The low catalyst loading combined with its high activity at low overpotential results in significant improvement on the onset potential for photoelectrochemical water oxidation. This transparent catalyst further enables the preparation of a stable hematite/perovskite solar cell tandem device, which performs unassisted water splitting.

EPFL2016

Nanostructured Electrocatalysts for Water Splitting Reactions

Lucas-Alexandre Stern

At the dawn of the 21st century, mankind is facing an important energy challenge. Future energy demand can only be met by large scale exploitation of renewable energy resources. Solar energy is abundant, with a sufficient capacity for the growing energy demand. However, it is necessary to implement renewable energy storage means. Hydrogen is considered as the energy carrier of the future. An energy economy based on hydrogen is viable only if hydrogen is produced from sustainable means. Water electrolysis is the most promising technique to produce hydrogen directly from water. Yet, the efficiency is limited and electrocatalysts are required to drive both the hydrogen evolution reaction and oxygen evolution reaction. Scarce and expensive materials are currently used to drive these reactions. It is, thus, imperative that efficient Earth-abundant catalysts are developed. Nanostructuring enhances the performance of inexpensive materials for water splitting and several catalysts were studied for this goal. Chapter 2 describes the application of nanostructuring technique to the archetypical catalysts that are nickel oxides for oxygen evolution. The prepared nanoparticles show superior activity towards oxygen evolution than that of their bulk counterpart. The ultrafine size of nanoparticles synthesized allowed the exposure of a higher number of active sites. The activity for oxygen evolution was, thus, significantly enhanced. We also detailed that short conditioning of the electrode improved the performance of the evaluated materials. In chapter 3 we carefully studied nanoparticles and nanowires of nickel phosphide. The catalysts is known to be really active for hydrogen evolution. Our group suspected that this material was, also, a potential oxygen evolution catalyst. We proved, for the first time, that nickel phosphide is a remarkable oxygen evolution catalyst. Under alkaline conditions, used for oxygen evolution, we observed an in-situ formation of a core-shell heterostructure, a typical nanostructure architecture. The surface oxidation of this material allowed high oxygen evolution capabilities. We concluded that careful oxidation of nanostructured materials, as observed for nickel phosphide, is essential for the preparation of future outstanding oxygen evolution catalysts. Chapter 4 details the fabrication of direct solar-to-fuel electrode using cobalt phosphide as hydrogen evolving catalyst. Instead of using the electrical energy provided by solar energy, solar irradiation on the developed assembly allows direct hydrogen production. The careful design of the light-sensitive electrode (photocathode) is detailed. Simple incorporation of cobalt phosphide by means of photodeposition alleviates the cost of the electrode fabrication. The assembled electrode is active for hydrogen evolution under visible light irradiation. In the chapter 5 we sought to fabricate a 3-dimensional hydrogen evolving catalyst. This nanostructure is based on polymer brushes on a flat conducting substrate. Once the brushes are grown on the substrate, a molybdenum sulfide catalyst is then incorporated by simple soaking technique. We were able to tune the loading of the catalyst and the corresponding activity by changing the polymer properties. The height of the polymer was modified as well as its packing density. The resulting activity proved superior to similar approaches and to previous reports on carefully engineered molybdenum sulfide catalysts.

EPFL2017

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

Antoine Philippe Van Muyden

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.