Nature 491, 235–239 (2012)

Dwindling resources of crude oil and its increasing price, in addition to the threat of climate change, have all resulted in a greater focus on renewable fuels — particularly biofuels, which are the result of biological carbon fixation. The most common biofuels produced are ethanol and butanol, but these can be difficult to integrate into the existing fuel-use infrastructure. Also, although biological production of higher hydrocarbons has been demonstrated, these routes tend to suffer from poor yields and titres (the amount of fuel produced per volume of fermentation broth). Now, Dean Toste and co-workers from the University of California, Berkeley, have proposed a chemical method that could be used to upgrade more common and higher-yielding fermentation products into a mixture containing the desired hydrocarbons.

Clostridium acetobutylicum is a bacterium that has been known for almost 100 years, and it produces, by fermentation, a mixture of acetone, butanol and ethanol (widely known as ABE). Toste and co-workers recognized that this classic mixture of two-, three- and four-carbon products harbour both electrophilic and nucleophilic carbon atoms such that a suitable chemical process might enable conversion to higher hydrocarbons. The reactions involved are relatively simple — a combination of transition-metal-catalysed dehydrogenation of the alcohols, base-catalysed aldol reactions, dehydration and a final reduction result in a formal alkylation of the acetone. The key, however, is to optimize the process to avoid the multitude of possible side reactions and obtain — depending on the reaction conditions — linear ketones with between five and eleven carbon atoms (pictured).

Development of these reaction conditions into a process to produce various hydrocarbon fuels also requires integration with the fermentation system. Here, Toste and co-workers have designed a two-phase liquid-extraction system that allows the ABE products to be easily separated from the fermentation broth for direct input into the catalytic upgrade system. The overall process converts up to 58% of the carbon in glucose into ketones. A final step in the process will be the integration with established procedures for the deoxygenation of the ketones.