Light-Induced Routes to Sustainable Biocrudes for Fuels and Lubricant Oils

Abstract: Fossil-based hydrocarbons are at present the ideal compounds for jet fuels and lubricant oils, so their replacement by novel technologies is not easy. Instead, sustainable routes to hydrocarbons, such as sunlight-driven processes, are desired to reduce the environmental impact by the transport sector. Photosynthetic microorganisms can convert water and CO2 into small hydrocarbons, yet, a second step is needed to convert them into jet fuels and lubricant oils. The aim of this thesis is to investigate photochemical routes for this second step.We first explore the triplet photosensitized dimerization of isoprene produced by cyanobacteria. We developed a combined photobiological-photochemical route from CO2 to C10 jet fuel via isoprene that has a climate change impact 80% lower than that of fossil jet fuel. The photosensitizer 1,1-dinaphthylmethanone absorbs in the near-UV light, so natural sunlight can be used with low photosensitizer loading (0.1 mol%). Later, other small conjugated dienes were investigated, providing a deeper understanding of the photodimerization. We concluded that isoprene is the ideal diene to be dimerized into jet fuel, as it has a suitable boiling point that facilitates its harvesting and as it dimerizes more efficiently than the other small volatile dienes.The photodimerization is then expanded to larger substrates to produce lubricant oils and diesel-like fuels. We found that α-phellandrene dimerizes very efficiently (>90%, 12h), and we utilize it in a cross-dimerization with less reactive monoterpenes and with isoprene. We also investigated the influence of light intensity in the reaction of α-phellandrene and the rates of triplet quenching of the photosensitizer by different monoterpenes.A final part of this thesis addresses the need of photochemical routes that can oligomerize unsaturated hydrocarbons other than conjugated dienes. The seminal idea is to use photoacids as catalysts. This journey started by designing a new photoacid based on the anilinium and dibenzotropylium cationic moieties. We found computationally a strong photoacidity, with pKa = -12. We discovered that the photoacidity is operating by a novel mechanism involving a reorganization of charge distribution within the dibenzotropylium moiety upon excitation, which interacts electrostatically with the anilinium moiety and makes the acidic proton of the anilinium unit more acidic.The work described in this thesis provides further understanding of the triplet photosensitized reactions first reported in the early 1960s, and it applies this important organic photoreaction in the context of renewable energy. Furthermore, the last part of this thesis contributes to new interpretations of photoacidity.