A Combined Photobiological-Photochemical Route to C10 Cycloalkane Jet Fuels from Carbon Dioxide via Isoprene

The hemiterpene isoprene is a volatile C₅ hydrocarbon, with industrial applications. It is generated today from fossil resources, but can also be made in biological processes. We have utilized engineered photosynthetic cyanobacteria for direct, light-driven production of bio-isoprene from carbon dioxide, and show that isoprene in a subsequent photochemical step, using simulated or natural solar light, can be dimerized into limonene, paradiprene, and isomeric C₁₀H₁₆ hydrocarbons (monoterpenes) in very high yields (above 90% after 44 hours) under sensitized conditions. The optimal sensitizer in our experiments is di(naphth-1-yl)methanone which we can use with a loading of merely 0.1 mol%, and it is easily recycled for subsequent photodimerization cycles. The isoprene dimers generated are a mixture of [2+2], [4+2] and [4+4] cycloadducts, and after hydrogenation this mixture is nearly ideal as a jet fuel drop-in. Importantly, the photodimerization can be carried out at ambient conditions. The high content of hydrogenated [2+2] dimers in our isoprene dimer mix lowers the flash point below the threshold (38 °C), yet, these dimers can be converted thermally into [4+2] and [4+4] dimers. When hydrogenated these monoterpenoids fully satisfy the criteria for drop-in jet fuels with regard to energy density, flashpoint, kinematic viscosity, density, and freezing point.