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 either near-UV or simulated or natural solar light, can be dimerized into limonene, paradiprene, and isomeric C10H16 hydrocarbons (monoterpenes) in high yields under photosensitized conditions (above 90% after 44 hours with near-UV and 61% with simulated solar light). The optimal sensitizer in our experiments is di(naphth-1-yl)methanone which we use with a loading of merely 0.1 mol%. It can also easily be 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 drop-in jet fuel. Importantly the photodimerization can be carried out at ambient conditions. However, 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. Life-cycle assessment results show a potential to produce the fuel in an environmentally sustainable way.
Three major changes/extensions have been made: 1: We have carried out a life-cycle assessment (LCA) which shows that our biofuel will lead to an extensive reduction in GHG emissions when compared to fossil-based aviation fuels. We have also identified that the main contributor to the GHG emission from our emerging technology comes from the production of the sodium nitrate used in the photobiological step. 2: We have also scaled up the photochemical dimerization from 10 mL scale to 400 mL scale. 3: We have developed a new cyanobacterial strain with improved genetic stability, by introducing the genetic constructs into the cyanobacterial chromosome, enabling long term productivity from the new engineered strain. The new strain has further been used in production experiments with intermittent removal of the gas phase, demonstrating enhanced production when product inhibition is avoided.
Rana etal ESI 220510FINAL