Scientists resurrect process to convert sugar directly to diesel
or winter driving conditions in different states,” said Blanch.
The process is versatile enough to use a broad range of renewable starting materials, from corn sugar (glucose) and cane sugar (sucrose) to starch, and would work with non-food feedstocks such as grass, trees or field waste in cellulosic processes.
“You can tune the size of your hydrocarbons based on the reaction conditions to produce the lighter hydrocarbons typical of gasoline, or the longer-chain hydrocarbons in diesel, or the branched chain hydrocarbons in jet fuel,” Toste said.
First World War process
The fermentation process, dubbed ABE for the three chemicals produced, was discovered by Weizmann around the start of the First World War in 1914, and allowed Britain to produce acetone, which was needed to manufacture cordite, used at that time as a military propellant to replace gunpowder. The increased availability and decreased cost of petroleum soon made the process economically uncompetitive, though it was used again as a starting material for synthetic rubber during the Second World War. The last U.S. factory using the process to produce acetone and butanol closed in 1965.
Nevertheless, Blanch said, the process by which the Clostridium bacteria convert sugar or starch to these three chemicals is very efficient. This led him and his laboratory to investigate ways of separating the fermentation products that would use less energy than the common method of distillation.
They discovered that several organic solvents, in particular glyceryl tributyrate (tributyrin), could extract the acetone and butanol from the fermentation broth while not extracting much ethanol. Tributyrin is not toxic to the bacterium and, like oil and water, doesn’t mix with the broth.
Brought together by the EBI, Blanch and Clark found that Toste had discovered a catalytic process that preferred exactly that proportion of acetone, butanol and ethanol to produce a range of hydrocarbons, primarily ketones, which burn similarly to the alkanes found in diesel.
“The extractive fermentation process uses less than 10 percent of the energy of a conventional distillation to get the butanol and acetone out – that is the big energy savings,” said Blanch.
“And the products go straight into the chemistry in the right ratios, it turns out.”
The current catalytic process uses palladium and potassium phosphate, but further research is turning up other catalysts that are as effective, but cheaper and longer-lasting, Toste said. The catalysts work by binding ethanol and butanol and converting them to aldehydes, which react with acetone to add more carbon atoms, producing longer hydrocarbons.
“To make this work, we had to have the biochemical engineers working hand in hand with the chemists, which means that to develop the process, we had learn each other’s language,” Clark said. “You don’t find that in very many places.”
Clark noted that diesel produced via this process could initially supply niche markets, such as the military, but that renewable fuel standards in states such as California will eventually make biologically produced diesel financially viable, especially for trucks, trains and other vehicles that need more power than battery alternatives can provide.
“Diesel could put Clostridium back in business, helping us to reduce global warming,” Clark said. “That is one of the main drivers behind this research.”
Coauthors of the study include former post-doctoral fellow Pazhamalai Anbarasan, graduate student Zachary C. Baer, postdocs Sanil Sreekumar and Elad Gross and BP chemist Joseph B. Binder.
— Read more in Pazhamalai Anbarasan et al., “Integration of chemical catalysis with extractive fermentation to produce fuels,” Nature 491 (8 November 2012): 235–39 (doi:10.1038/nature11594)
