Chemical industry, white biotechnology and renewables
Although today’s biofuel industry relies mainly on ethanol bioproduction, there is a growing concern that this molecule is not an optimal fuel. Ethanol has a low energy density; its production requires a costly distillation step; its generalization would require the adaptation of industrial equipment and car engines. The direct bioconversion of renewable resources into hydrocarbons compatible with the current petroleum industry appears as a more promising option. However, natural organisms have not evolved metabolic pathways for the efficient production of large amounts of such hydrocarbons.
The design and reduction to practice of an artificial metabolic pathway for producing the gaseous hydrocarbon isobutene will be presented. This pathway features the highest carbon balance allowed by thermodynamics and requires no consumption of reduced NAD(P) equivalent. It involves several enzymatic activities that are not known to operate in natural species.
Engineering of the enzymes and optimisation of the metabolic background are ongoing so as to enable the large-scale production of isobutene from renewable resources such as saccharose, starch-derived glucose, or fermentable sugars obtained from the degradation of lignocellulosic material, according to the general equation: C6H12O6 C4H8 + 2CO2 + 2H2O.
Gaseous bioproduction bypasses the limitations encountered in classical bioprocesses that lead to liquid products: there is no product inhibition, as the product does not accumulate in the vessel; there is no costly downstream processing, such as distillation or solvent extraction.
Isobutene can be further transformed into various liquid fuels compatible with current petrochemical infrastructure and engines. Such fuels include iso-octane (to be used as gasoline), iso-dodecane (to be used as jet fuel), iso-hexadecane (to be used as diesel) and ETBE (a widely used gasoline additive), by condensation with bioethanol.
The isobutene product tree also encompasses butyl rubber (a main component of tires), methyl-metacrylate (the precursor of organic glass), isobutene polymers ranging from lubricants to thermoplastics, and several fine chemical compounds.
The development of completely artificial metabolic pathways opens up new perspectives for bioprocesses in the domain of fuels and commodities, expected to play a major role in this forthcoming post-oil era.