This thesis investigates the catalytic upgrading of bio-oil to optimize its composition for fuel applications, with a focus on improving the stability and selectivity of bio-based hydrocarbons. Catalytic upgrading plays a crucial role in converting bio-oil into high-quality liquid fuels, ensuring improved stability and performance in energy applications. This research also explores the reaction pathways of oxygen-containing compounds during catalytic bio-oil upgrading, aiming to enhance selectivity and fuel quality. This research also explores the reaction pathways of oxygen-containing compounds during catalytic bio-oil upgrading, aiming to enhance selectivity and fuel quality.
Bio-oil, derived from lignocellulosic biomass, contains high levels of oxygenated compounds, which hinder its direct use as transportation fuel. These oxygenated compounds contribute to instability and low heating value, necessitating effective catalytic upgrading strategies. Catalytic upgrading provides a viable pathway to selectively remove oxygen while preserving the hydrocarbon framework, improving fuel properties. However, the high oxygen content in bio-oil (~35-50 %) poses a significant challenge for refining, leading to poor thermal stability and high acidity. ... mehrOxygenated species lead to undesired side reactions, catalyst deactivation, and increased coke formation during upgrading processes. Currently, hydrodeoxygenation (HDO) is the most widely applied catalytic upgrading technique to improve bio-oil quality. However, oxygen removal often leads to excessive hydrogen consumption and undesired cracking reactions, reducing carbon efficiency. This thesis investigates novel catalytic strategies to selectively remove oxygen while maximizing hydrocarbon retention, enabling efficient bio-oil upgrading.
Chapter 1 and Chapter 2 provides a comprehensive overview of the background on catalytic upgrading of bio-oil to improve its quality for fuel applications. The chapter introduces the key challenges associated with bio-oil, including its high oxygen content, poor stability, and incompatibility with conventional petroleum-based fuels. Various catalytic processes for bio-oil upgrading are discussed, highlighting their advantages and limitations. Furthermore, the fundamental reaction pathways of catalytic HDO and its impact on the removal of oxygen-containing species are explored. In addition, the chapter introduces the role of different catalyst types (e.g., metal-supported catalysts, bifunctional catalysts) and their performance in deoxygenation reactions. The influence of reaction conditions such as temperature, pressure, and hydrogen supply is also reviewed. Finally, the existing research gaps in catalytic bio-oil upgrading are identified, and the key objectives of this thesis are outlined.
In the Results and Discussion (Chapter 4), Part I (Section 4.2) investigates the critical role of HDO in the efficient utilization of biomass, with a particular focus on ruthenium (Ru) catalysts. Since oligomeric species play a significant role in bio-oil upgrading, this chapter employs a dimer model compound to elucidate the reaction mechanism, providing insights for the rational design of catalysts in subsequent studies. Initially, the adsorption behavior of 2-Phenylethyl phenyl ether (PPE) on the Ru (0001) surface was examined using density functional theory (DFT) calculations, leading to the determination of its optimized adsorption configuration. Subsequently, the reaction pathways for HDO on the Ru catalyst were computationally analyzed, offering mechanistic insights into the transformation of dimeric lignin-derived compounds. Building upon these theoretical findings, the catalytic performance of Ru/Nb2O5 was evaluated in the HDO of PPE, demonstrating high efficiency in oxygen removal. To further assess its practical applicability, the catalyst was tested in the HDO of real bio-oil, allowing for a more comprehensive evaluation of its effectiveness under complex reaction conditions. This study systematically investigates the HDO of PPE over a Ru-based catalyst, illustrating the potential of employing dimer model compounds in mechanistic studies and highlighting the promising capabilities of Ru-based catalysts for bio-oil upgrading.
Building on the findings of Part I (Section 4.2), which identified C-O bond cleavage in β-O-4 linkages as the key step, a series of catalysts were designed for β-O-4 hydrogenolysis. Part II (Section 4.3) investigates the catalytic depolymerization of biomass oligomers, a crucial step in biomass valorization. Due to the stringent conditions required for their conversion into high-value monomers, Ni-based catalysts derived from layered double hydroxides (LDH) were developed to enhance depolymerization efficiency under mild conditions. The catalytic performance of these Ni/Al-LDH-derived catalysts was evaluated using a biomass dimer model compound, achieving complete conversion at 125 °C and 25 bar. Characterization techniques, including TEM and XPS, confirmed the coexistence of Ni(0) and Ni(II) species on the catalyst surface. DFT calculations revealed that Ni(II) modifies the d-band center of Ni(0), influencing its adsorption behavior and catalytic activity. The Ni/NiO interface also displayed superior thermodynamic and kinetic properties compared to Ni (111). This chapter highlights the synergistic role of Ni(0) and Ni(II) in lignin depolymerization, providing insights for the rational design of efficient Ni-based catalysts under mild conditions.
The designed catalysts exhibited high C-O bond cleavage activity In Section 4.3, yielding primarily oxygenated monomers. However, their limited deoxygenation capability indicated the need for further optimization to achieve efficient oxygen removal. Part III (Section 4.4) builds upon these findings by incorporating oxophilic metal dopants into the Ni-based catalysts to enhance their deoxygenation performance. A series of Ni/Al2O3 catalysts doped with Mo, Ce, Nb, and W was synthesized using an interlayer doping strategy based on layered double hydroxide (LDH) structures. These modifications significantly improved the catalysts' ability to remove oxygen-containing functional groups. Guaiacol, a widely used model compound for lignin-derived bio-oil, was employed to assess catalytic performance, achieving complete conversion and 99% selectivity toward cyclohexane under optimized conditions. Notably, the doped catalysts also demonstrated superior efficiency in deoxygenating crude bio-oil, outperforming commercial noble metal-based Ru/C catalysts. Mechanistic investigations integrating DFT calculations and experimental characterization revealed that MoOx clusters on Ni surfaces reduce the C-O bond cleavage energy barrier to 0.40 eV, significantly lower than that of conventional Ni catalysts. This enhancement underscores the synergistic effect of oxophilic metal dopants, which facilitates oxygen removal while maintaining catalytic stability. This chapter provides an advanced catalyst design strategy for bio-oil upgrading, offering a cost-effective alternative to noble metal catalysts and deepening the fundamental understanding of HDO mechanisms in biomass valorization.
In Section 4.4, the catalytic upgrading strategies effectively achieved oxygen removal, yet predominantly produced oxygen-containing monomeric compounds with limited carbon chain lengths. Such upgraded bio-oils typically cannot satisfy the requirements for high-value transportation fuels such as sustainable aviation fuel (SAF) or biodiesel. To overcome this limitation, Part IV (Section 4.5) explores the utilization of biomass-derived syngas (CO/H2) as an external carbon source to enhance the carbon chain length of upgraded bio-oil hydrocarbons through catalytic C–C coupling. A series of Ru/β-zeolite catalysts were developed and evaluated for their capability to promote carbon-chain elongation using guaiacol as a representative phenolic compound. Among the tested catalysts, Ru/β-zeolite demonstrated exceptional catalytic activity, achieving effective C–C coupling and chain elongation under moderate reaction conditions. Furthermore, application of this catalyst to real bio-oil confirmed its high catalytic efficiency, successfully extending the carbon chain length and significantly improving fuel properties. Characterization techniques, including X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX), revealed that the zeolite support maintained its crystalline structure, while Ru nanoparticles were uniformly dispersed, providing abundant active sites. This study demonstrates the potential of employing syngas-assisted catalytic upgrading strategies to produce high-quality renewable fuels, offering valuable insights for sustainable biomass valorization and the development of advanced catalytic processes for biofuel production.
Chapter 5 summarizes the primary findings and key contributions of this thesis, highlighting the advancements made in catalytic HDO for biomass-derived feedstocks. The thesis provides a deeper understanding of catalytic mechanisms and illustrates effective strategies for optimizing catalyst performance to enhance bio-oil upgrading efficiency. The chapter also identifies directions for future research, emphasizing the need to further investigate catalyst stability, recyclability, and industrial scalability. It suggests exploring catalyst performance in continuous-flow systems and conducting more in-depth mechanistic studies on catalyst-substrate interactions. These efforts could significantly contribute to the development of practical, sustainable, and economically feasible biofuel production processes.
