Catalytic Upgrading of Biomass-Derived Carboxylic Acids to Fuels and Chemicals

Our reliance on fossil fuels, which is unsustainable due to dwindling reserves, has led to various environmental issues. Lignocellulosic biomass could serve as a renewable source of carbon and energy for the production of fuels and chemicals. While several valorization routes of biomass-derived sugars (via furans or sugar alcohols) exist, the upgrading of the biological carboxylate platform is relatively unexplored, mainly due to historical focus on ethanol production via fermentation. Research on biomass-derived carboxylic acids could increase our knowledge on carboxylic acid upgrading and open up new avenues towards sustainable production of fuels and chemicals from lignocellulosic biomass.

The production of olefins from carboxylic acids via tandem hydrogenation/dehydration was carried out, with olefin selectivities > 90% at close to 99% conversion of carboxylic acids using a Cu/SiO2-Al2O3 catalyst. Interestingly, a sudden selectivity switch from olefins to predominantly alkanes was observed at full conversion. Using various intermediate products and catalyst surface studies, this selectivity switch phenomenon was ascertained to originate from the adsorption of small amounts of carboxylic acid on the catalyst surface, which prevented the binding and subsequent overhydrogenation of olefins. This finding suggests that carboxylic acids has potential to be used in influencing product distributions of other catalytic reactions, especially if selective double bond preservation is required. This phenomena could also be used to explain the difference in catalyst performance observed when using commercial model compounds versus real biomass-derived feed, given the prevalence of carboxylic acid impurities in biomass-derived streams. This route represents a single-step upgrading of carboxylic acids to olefins with no loss in carbon efficiency and without use of expensive stoichiometric reagents, which is important for economical production of bulk chemicals from lignocellulosic biomass.

In parallel, a study on carboxylic acid upgrading via single-step ketonization/cascade aldol condensation to an aviation fuel additive was performed over Cu/ZrO2. Biomass-derived acetic acid and butanoic acid was converted to an organic oil composed of aromatics and cycloalkenes with a carbon number range of C8 - C16 at mass yields of around 20 wt %. By-products include gaseous hydrocarbons (mainly propylene) and water resulting from condensation. This organic oil represents up to 96% deoxygenation of the starting carboxylic acids, and has been tested to be compatible as a 10 vol % blend with Jet A-1 fuel in terms of specific energy and distillation properties. While previous research on ketonization and aldol condensation have performed the reactions separately, this route combines both in a single step to upgrade short carboxylic acids to long chain hydrocarbons, while valorizing the often neglected acetate fraction of biomass. This study shows a combination of C-C coupling and extensive deoxygenation, both crucial towards successful upgrading of lignocellulosic biomass to fuels.

Production, characterization and upgrading of biomass-derived acetal-stabilized carbohydrates

Ydna Marie Questell-Santiago

Biomass is an attractive source of renewable carbon-based fuels and chemicals. Multiple research efforts are centered around integrated biorefineries, where all fractions of biomass are converted to fuels or chemical products. These efforts are focused around making biorefineries more economically competitive and sustainable. One of the major drawbacks is the incomplete utilization of biomass, specifically, lignocellulosic biomass, which is the largest terrestrial source of renewable carbon. Given the significant differences in the structure and reactivity of its components, the integrated depolymerization and subsequent upgrading of all the fractions involved has been challenging. The depolymerization of lignocellulosic biomass is controlled by the kinetic competition between depolymerization and degradation reactions. Multiple strategies have relied in the modification of reaction conditions to mitigate this limitation, but are either expensive or have not fully overcome this issue. Here, I present the use of protecting group chemistry to reversibly stabilize biomass-derived carbohydrates during depolymerization by acetal formation with formaldehyde (FA). This stabilization technique, altered the kinetic paradigm by limiting sugar dehydration and further degradation. The depolymerization of beech wood using acid organosolv processes in the presence of FA led to recoveries of over 90% sugars with a final concentration of ~5wt%. The produced acetal-stabilized biomass-derived sugars possess versatile properties compared to unmodified sugars, which motivated me to further investigate their reactivity. I found that diformylxylose (acetal-stabilized xylose, DX) produced an intermediate during its acid hydrolysis deprotection that promoted a 1,2-hydride shift that led to the low temperature production of furfural. Generally, this shift is catalyzed by Lewis acids, but its presence did not affect the rate of reaction when DX was used as a starting material. Based on these findings and Operando NMR studies, a tandem hydrolysis-dehydration of DX to furfural, was pro-posed. The benefits of using acetal-stabilized sugars was studied by performing a preliminary techno-economic analysis for the non-enzymatic production of ethanol and the direct upgrading of DX to xylitol. After taking account the solvent savings and the extra recovery steps of FA, a minimum selling price of

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2014) of gasoline gallon equivalent was calculated. In the case of xylitol production, a proposed process was constructed around the volatility and unique reactivity of DX. The key advantages included the purification potential of DX by distillation rather than activated carbon adsorption and ion exchange chromatography, which is used for xylose purification, and a rate limiting step that was independent of hydrogen pressure, which minimized hydrogen requirements. The on stream study of the catalyst demonstrated an evolution of the active sites of Pt that led to an enhanced xylitol yield. This phenomenon is attributed to the formation of Pt(OH)2 (detected by X-ray photoelectron spectroscopy), which accelerates the hydrolysis and hydrogenation reactions due to the proximal acid site to Pt0. In overall, I have demonstrated that acetal functionalization of carbohydrates can, not only increase yields by reducing sugar degradation but also reveal new reactivity that can be exploited to create novel catalytic upgrading pathways for the production of bio-based chemicals.

EPFL2019

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

Methodology for the identification of promising integrated biorefineries

Ayse Dilan Celebi

Steady increase in global energy consumption, greenhouse gas emissions, and depletion of fossil energy resources, together with increased attention towards sustainable development has prompted researchers to discover economical and environmentally competitive non-petroleum alternatives and biomass is considered to be one of the most promising renewable sources of energy and carbon of this matter to address the aforementioned issues within a biorefinery platform. A biorefinery is an integrated processing facility that converts biomass into transportation fuels, value-added chemicals, heat, and electricity via biochemical and thermochemical conversion routes. Integrated biorefineries are capable of mimicking petroleum refineries via application of advanced process synthesis methods including modeling, simulation, integration and optimization techniques. This thesis presents a comprehensive synthesis methodology for the integration of bioprocessing technologies in biorefineries by addressing techno-economic and environmental sustainability analysis. The proposed systematic design approach combines advanced process modeling approaches, process integration techniques, and multi-objective optimization algorithms to assess the performance of the overall system with respect to economic, environmental, and energetic indicators. The design strategy is applied based on different case studies. Results indicate that multi-product processes can yield significant cost and environmental benefits. Additionally, the integration of biochemical and catalytic processes with thermochemical conversion pathways results in increased carbon efficiency and economic and environmental competitiveness. Synergies between biorefineries and energy system are assessed by integrating industrial plants with a cogeneration system producing biofuels and process heat. In doing so, system efficiency is increased by coupling the proposed cogeneration system with carbon capture and sequestration (CCS) and power-to-gas technologies to store the renewable intermittent electricity in the form of biofuels. The results exhibit that through system expansion, integrated biorefinery systems allow us to imitate fossil refineries with a large spectrum of bio-based products. This further increases the resource efficiency while offering a promising solution to mitigate CO2 emissions and hence reaching the longer-term decarbonization target set by the Paris Agreement.

EPFL2019

Coupling of electrocatalysts to photoabsorbers for solar fuels production

Carlos Gilberto Morales Guio

EPFL2016

Production, characterization and upgrading of biomass-derived acetal-stabilized carbohydrates

Ydna Marie Questell-Santiago

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EPFL2019

Methodology for the identification of promising integrated biorefineries

Ayse Dilan Celebi

EPFL2019