The hemiterpene isoprene is a volatile C<sub>5</sub>
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 C<sub>10</sub>H<sub>16</sub> 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.
Synthesis of Diesel and Jet Fuel Range Cycloalkanes with Cyclopentanone and Furfural