Production of Sustainable Aviation Fuels by CO₂ valorisation via combined fermentation and chemical catalysis

Aviation stands out as one of the sectors with the highest impact in greenhouse gas (GHG) emissions due to their intensive reliance on fossil resources. This contribution is only expected to increase given the lack of foreseeable replacements for the fossil origin of its fuels, not easily attainable by other means due to the complex regulations and specifications they require. Therefore, increasing efforts are being made in order to obtain the same compounds present in traditional jet fuels albeit with a renewable origin. The present PhD Thesis thus aims at developing the foundations for a process that would produce the different components of jet fuel mixtures from CO₂ and from renewable biomass, by combining two fields of science: biotechnology and chemistry. The idea consists of a four-step process, where CO₂ is initially uptaken by a thermophilic acetogen along with (green) hydrogen. This acetogen produces acetate, to be consumed by a second thermophilic bacteria (Parageobacillus thermoglucosidasius) that, after appropriate metabolic engineering, provides different ketones (acetone, butanone) and alcohols (ethanol, isopropanol, 1-butanol). These fermentation-derived oxygenates are catalytically upgraded, first by alkylation where ketones are elongated with alcohols resulting in longer-chain ketones, followed by hydrodeoxygenation (HDO) where the oxygen atoms are removed as water finally obtaining hydrocarbons matching those present in traditional aviation fuels.
Work performed on the biotechnology section of the Thesis focused on the metabolic engineering of P. thermoglucosidasius strains capable of producing 1-butanol. In Chapter 2, the six genes from the classical butanol-producing pathway from Clostridium acetobutylicum were introduced in two different operons within the genome under the control of an inducible Ptet promoter system. Among the different developed strains, including non-sporulating variants, the maximum butanol production was obtained for P. thermoglucosidasius Btb-Ptet, with a final titre of 0.375 g/L. This strain was then selected for up-scaling into 2 L bioreactors in Chapter 3, where different process parameters and substrate mixtures were assessed looking into their effect on butanol production. The amount of dissolved oxygen (DO) was found to be a crucial factor, as butanol production was only observed when working under limited aeration conditions obtained by relaxing the DO control. The strain was capable of growing in the presence of up to 20 g/L glucose and 10 g/L acetate; however, the observed titres of butanol did not reach those of the previous chapters as it was stripped through the off-gas of the fermenter. Finally, in Chapter 4 several metabolic optimisation strategies were followed with the aim of improving the product yield. Each of the different enzymes involved were further overexpressed individually, with increased titres being obtained when increasing the activity of thiolase (Thl) and crotonase (Crt): butanol titres compared to the control were increased by 8.4% and 30.8%, respectively. An alternative pathway through butyrate was assessed based on a native pathway already present in the bacteria; while the production was not improved regarding the parental strain, the feasibility of this pathway was confirmed, and further improvement can be expected by testing alternative thermostable enzymes. Butanol production can be enhanced by focusing on addressing the high requirement of the pathway for reducing NADH equivalents, which could be solved by tuning the expression of NADH-regeneration systems.
Upgrading butanol along with the rest of oxygenates synthesised by P. thermoglucosidasius was the focus of the chemistry section of the Thesis. A thorough literature review was performed in Chapter 5, where the mechanisms for the catalytic upgrading of these compounds by means of alkylation and HDO were analysed. Palladium catalysts were observed to be the most efficient catalysts for this purpose given its high hydrogenation capabilities, while other alternative metals were also addressed owing to the high price and environmental impact associated to Pd. The benchmark catalyst, Pd/C, along with K₃PO₄ as base, were used for a comprehensive study and kinetic modelling of the alkylation process in Chapter 6. The alkylation of acetone with butanol towards 6-undecanone (C11), as representative oxygenates, was optimised aiming at the best combination of parameters: temperature, catalyst loading and base loading. An unprecedented reaction selectivity towards C11 of 70.2% was obtained under the optimised conditions of 150°C, 0.5 wt% Pd/C loading and 15 wt% K₃PO₄  loading. These parameters were applied to different combinations of oxygenates in Chapter 7, and different long-chain ketones (both linear and branched) were obtained, as precursors to two of the fractions that comprise aviation fuels. Most notably, dialkylation was observed on butanone, which was not expected given the presence of only one methyl group in the ketone. Finally, several Pd-zeolite catalysts were synthesised and tested for HDO on the ketones derived from alkylation of acetone with butanol. Product selectivities of 49% and 76% towards heptane and undecane (an existing component in traditional jet fuels) were obtained, respectively.