Ethanol has dominated the biofuel industry’s attention over the past few years. But synthetic-biology companies are now scaling up production of what some say is a superior fuel: butanol, an alcohol with four carbon atoms to ethanol’s two. Compared to ethanol, butanol stores more energy per litre, is less corrosive to pipelines, is more easily separated from water and can be blended into gasoline (petrol) at higher concentrations before vehicle engines are damaged.
On 24 May, Gevo, a synthetic-biology firm based in Englewood, Colorado, announced the startup of the first commercial plant to chew up biomass to produce isobutanol. (The ‘iso’ means that this form of butanol has a carbon atom branched off the middle of a three-carbon chain). The factory, in Luverne, Minnesota, will be making 1 million US gallons (3.8 million litres) of isobutanol per month by the end of the year, with a maximum annual production capacity of 18 million gallons, and is a retrofitted bio-ethanol plant. Gevo saw its shares rise by more than 9% after the news and says it next plans to retrofit a plant in South Dakota (says Reuters).
The isobutanol will not be used as a fuel for now; rather, Gevo is selling most of its output to South African chemicals giant Sasol. (Butanol is variously used in paints, solvents and resins and as a starting point for synthesizing more complicated chemicals). That business strategy has been adopted by many other synthetic-biology firms that are selling their bio-products to chemicals manufacturers rather than trying to compete in the cheap fuels market.
Gevo’s technology, in part licensed from James Liao, a metabolic engineer at the University of California, Los Angeles, uses yeast cells that have been tweaked so that the metabolic pathways that would produce amino acids are diverted to isobutanol production (for a similar idea in E.coli bacteria, see S. Atsumi, T. Hanai & J. C. Liao Nature 451, 86–89; 2008). The company is still disputing technology rights with another bio-butanol company, Butamax, a joint venture between oil giant BP and chemicals firm DuPont.
A different four-carbon alcohol, n-butanol, is the target for the UK’s Green Biologics (n-butanol has all four carbon atoms in a line). The firm, which this January merged with Ohio company Butylfuel, uses an engineered strain of the Clostridia microbe that can chew up cellulosic and other waste feedstocks that don’t compete with food-supply needs.
It’s also looking to buy up and retrofit old ethanol plants in the United States, but right now has licensed its technology to biobutanol producers in China, such as Laihe Songyuan Chemical. China’s largest biobutanol plant is now making some 10 million US gallons a year, says Sean Sutcliffe, chief executive of Green Biologics.