Catalytic production of high-energy-density spiro polycyclic jet fuel with biomass derivatives

High-energy-density (HED) fuel (e.g. widely used JP-10 and RJ-4), featuring compact 3D polycyclic structure with high strain, is of critical importance for volume-limited military aircraft, since their high density and combustion heat can provide more propulsion energy. To reduce the reliance on petroleum source, it is highly desirable to develop renewable alternatives for the production of strained polycyclic HED fuel, but which remains a big challenge because of the inaccessibility caused by the high strain. We herein demonstrate a three-step catalytic route towards highly strained C₁₇ and C₁₈ spirofuel with biomass feedstocks. The process includes catalytic aldol condensation of renewable cyclohexanone/cyclopentanone with benzaldehyde, catalytic spiro Diels-Alder (D-A) reaction of aldol adduct with isoprene, and catalytic hydrodeoxygenation. The key spiro D-A reaction is enabled by the catalysis of heterogeneous Lewis acidic ionic liquid. The chloroaluminate IL, formed by benign urea and AlCl₃, exhibits good catalytic performance and reusability for this step. An eventual hydrodeoxygenation (HDO) over Pd/C and H-Y produces strained tricyclic spirofuel with density >0.93 g/mL, combustion heat >41 MJ/L and freezing point <  −40 °C, which are better than the properties of tactical fuel RJ-4. Therefore, it is anticipated that the as-prepared renewable fuels have the potential to replace traditional petroleum-derived HED fuels.