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

## TL;DR

This paper introduces a new type of sustainable fuel made from renewable sources that matches the energy density of traditional fuels but is safer and more stable.

## Contribution

The first class of high-energy-density fuel candidates made from bioderived fatty esters using polycyclopropanated lipid-inspired ionic liquids.

## Key findings

- PCP-ILs achieve energy densities of about 30 MJ/L, comparable to aviation fuels.
- PCP-ILs have negligible vapor pressure and high flash points, improving safety and storage.
- Cyclopropyl moieties reduce melting points, enhancing fluidity and performance.

## Abstract

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.