Sustainable aviation fuel-range intermediates from self-aldol condensation of cyclohexanone using low-cost niobium phosphate catalyst

Biomass-derived chemical feedstocks are sustainable precursors for producing the range of compounds found in conventional hydrocarbon fuels. For instance, pyrolysis of lignin can produce high yields of phenolic compounds, which in turn can be quantitatively converted into cyclic ketones such as cyclohexanone, a model precursor for energy-dense aviation fuel hydrocarbons. Producing aviation fuel range hydrocarbons from short-chain oxygenated compounds such as cyclohexanone, requires both carbon chain elongation and oxygen removal via deoxygenation chemistries. In this study, a range of high-density fuel precursor molecules with fuel relevant carbon numbers (C12 – C18), have been produced via solvent-free carbon-carbon coupling of cyclohexanone as a model lignin-derived ketone. In the experiments, niobium phosphate (NbOPO4) was synthesised and used as a solid acid heterogeneous catalyst. Batch reactions were carried out at 160 °C with 1h–3 h reaction times, under nitrogen or hydrogen atmospheres. Cyclohexanone conversions of between 68.4 % and 95.4 % were achieved depending on reaction atmosphere. High selectivity towards dimeric ketones, the primary products of aldol condensation of cyclohexanone, was obtained under hydrogen gas. Initial hydrogen pressure of 5 bar gave the highest selectivity (nearly 90 %) mainly towards the dimeric 2-cyclohexylidenecyclohexanone and 2-(1-cyclohexen-l-yl) cyclohexanone. In contrast, mostly alkyl aromatic hydrocarbons with >C12 carbon atoms (e.g., cyclohexylbenzene) were formed under nitrogen, indicating that the inert atmosphere promoted the dehydration and dehydrogenation of primary aldol products. Correspondingly, the Brønsted acid and Lewis acid sites of the NbOPO4 were enhanced under nitrogen, with 3.7 wt% coke formation compared to 0.9 wt% under hydrogen. This work highlights the effectiveness of a low-cost catalyst to produce high yields of compounds that can be converted, via further deoxygenation and hydrogenation, to potentially energy-dense liquid hydrocarbons within the sustainable aviation fuel range.