Polycyclopropanated Lipid-Inspired Ionic Liquids as High Energy-Density Fuel Candidates

Climate change necessitates the urgent development of sustainable alternatives to petroleum-derived fuels for energy-intensive applications such as aviation, rocketry, and long-haul transport, where electrification remains impractical. This study presents polycyclopropanated lipid-inspired ionic liquids (PCP-ILs) as the first class of high-energy-density fuel candidates synthesized from renewable bioderived fatty esters. Our design strategy draws inspiration from natural lipid structures, leveraging their inherent fluidity characteristics to create functional ILs. We developed a facile synthesis route using Cu-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry, which enables direct incorporation of a cyclopropyl ring onto the nitrogen-rich 1,2,3-triazolium headgroup in quantitative yields. The resulting PCP-ILs demonstrate remarkable properties essential for fuel applications, such as negligible vapor pressure, eliminating detectable boiling points, and expected high flash points that enhance safety and storage stability. Strategic placement of cyclopropyl moieties in both the cationic headgroup and aliphatic side chains, mimicking fluidity-conferring features in biological lipids, significantly reduces melting points compared to non-cyclopropanated analogues. Computational and X-ray crystallography studies systematically elucidate how molecular packing and structural organization enable significant melting/freezing point reduction. These PCP-ILs achieve theoretical volumetric energy densities of ca. 30 MJ/L, competitive with conventional aviation fuels, while providing superior safety profiles through negligible vapor pressure and enhanced thermal and chemical stability compared to unsaturated hydrocarbon alternatives. These combined properties demonstrate that bioinspired PCP-ILs can deliver the high technical performance required for demanding energy applications while maintaining sustainability advantages, establishing a pathway for renewable alternatives in difficult-to-decarbonize sectors.