Lignine als basis voor geavanceerde vloeibare biobrandstoffen voor de luchtvaart

Decarbonizing aviation remains one of the hardest challenges in the global energy transition, as the sector depends almost entirely on fossil-based fuels whose exceptional energy density and physicochemical properties are difficult to replicate with alternative energy carriers. To meet the European Union's climate neutrality objectives, the ReFuelEU Aviation Regulation mandates that 70% of aviation fuel be replaced by sustainable aviation fuel (SAF) by 2050. However, current SAF production is limited, as only a few ASTM-approved and mature processes exist—restricting the large-scale production of SAF. The approved routes mainly yield linear hydrocarbons (iso- and n-alkanes), leaving a deficit of cyclic components—naphthenes and aromatics—that are essential for reproducing the energy density and functional properties of conventional jet fuels. In this context, lignocellulosic biomass (LCB), and particularly its lignin fraction, represents a promising renewable feedstock to supply these cyclic hydrocarbons. Lignin is the largest natural source of aromatics on Earth; however, its high oxygen content makes it unsuitable as a drop-in fuel. To overcome this, its oxygen content can be effectively reduced through hydrodeoxygenation (HDO), a catalytic process that removes oxygen from lignin-derived methoxy phenols, yielding cyclic hydrocarbons within the jet-fuel range. This thesis advances the understanding of the lignin-to-SAF pathway across three complementary dimensions: feedstock availability, feedstock quality, and molecular-level reactivity. First, an assessment of European lignin availability demonstrated that the amount of LCB potentially harvestable under sustainable conditions could deliver the quantity of lignin necessary to meet future SAF demand. However, to achieve this sustainably, the gradual introduction of lignin-first valorization strategies into existing lignocellulosic biorefining pathways is essential. Among these, Reductive Catalytic Fractionation (RCF) emerges as a particularly promising approach, capable of producing around 20 Mt C per year of lignin oil—equivalent to approximately 30 Mt per year—sufficient to supply the targeted amount under the ReFuelEU Aviation Regulation of 5.6 Mt per year of aromatics and 10.2 Mt per year of naphthenes after HDO. Second, the thesis investigates the relationship between the physicochemical properties of lignin oil and HDO performance. Using various lignin oils upgraded over a phosphidated Ni/SiO2 catalyst, it was shown that RCF-derived oils, prepared from eucalyptus, poplar, and pine using Ru/C as the biorefining catalyst, significantly outperform both conventional technical lignins and RCF-derived lignin oils produced using Pd/C. The superior performance of Ru/C-derived lignin oils was attributed to their lower molecular weight compared to technical lignins—resulting in higher solubility in the alkane medium used for HDO—and to their lower γ-OH end-unit content compared to RCF-derived lignin with Pd/C as catalyst. A direct correlation between the γ-OH content, determined by 1H-13C HSQC NMR, and the selectivity toward jet-range hydrocarbons after HDO was established. Lignin oils with a lower γ-OH content, characteristic of those obtained from Ru/C-catalyzed RCF, exhibited markedly higher selectivity toward SAF-range naphthenes than Pd/C-derived oils. In contrast, the feedstock type (hardwood vs. softwood) had minimal influence, confirming that the biorefining catalyst and strategy are the key parameters governing jet-range hydrocarbon yields and selectivity. Finally, a fractionation-guided molecular investigation provided mechanistic insight into the conversion of RCF-derived lignin oils into SAF-range hydrocarbons. Lignin oil obtained from pine via Ru/C-catalyzed RCF was separated into six fractions (H100-E100) using solvent polarity-based extraction with n-heptane and ethyl acetate. Each fraction was comprehensively characterized by GPC, GCxGC-MS/FID, and 1H-13C HSQC NMR, and subsequently upgraded via HDO using phosphidated Ni/SiO2 at 300 °C and 50 bar H2 in dodecane. Early fractions (H100-H40) proved highly effective in producing C8-C18 naphthenic hydrocarbons, exhibiting a relative carbon yield (RCY) sequence of H100 > H80 > H60 > H40 (74%, 55.4%, 52%, 35.4%). The H100 fraction delivered predominantly monomer-derived mono-naphthenes, particularly propyl-cyclohexane (54 % RCY), while the heavier fractions (H80-H60) produced bi-naphthenes such as C14-C18 species, with C17 bi-naphthenes being the most abundant (17 % RCY in H60). These results demonstrate that monomer- and dimer-rich fractions are the most suitable for SAF production, whereas oligomeric fractions are poorly converted under HDO but may serve as valuable co-products for other applications—such as polymeric materials. Overall, this work establishes lignin as a viable renewable source of cyclic hydrocarbons for sustainable aviation fuel production. By integrating feedstock availability analysis, structure-property-performance relationships, and molecular-level transformation pathways, this thesis defines a technically grounded route toward scalable lignin-derived SAF aligned with European decarbonization targets.