Reaction & Synthesis

Isobutanol beckons as biofuel


Feb 04, 2008

Genetically modified bacteria promise to provide a low-cost way to produce isobutanol from sugar — opening up the prospect of the alcohol displacing ethanol as a biofuel, say developers at the University of California, Los Angeles. The bio-based route should halve the cost of making isobutanol, believes Pat Gruber, CEO of Gevo, Inc., Pasadena, Calif., which holds the exclusive license for the technology from UCLA.

Isobutanol boasts a higher energy density and octane number than ethanol as well as lower hygroscopicity, notes James C. Liao, professor of chemical and biomolecular engineering at the school. However, bacteria ordinarily won’t synthesize higher alcohols like isobutanol. Liao and his team overcame this through genetic modification of E. coli — shunting intermediates from amino-acid biosynthesis pathways to alcohol production. This results in conversion of glucose with high specificity and at high yield to isobutanol.

While Liao has produced the alcohol at the bench scale so far, the process itself is conventional. A reactor is seeded with the modified E. coli and glucose. Reaction over the next 40+ hours converts all the sugar into isobutanol and carbon dioxide, he notes. Yields, now 0.35 g of alcohol/g of sugar, already approach 90% of theoretical, and should at least reach 90% with optimization of the bacteria and the process, says Liao.

Gevo now is working to optimize the organism and the route. A 1,500–2,000 L/batch pilot plant should be operating within six months, says Gruber, adding that the process should be ready for commercialization by the end of 2009 or early 2010.

The first installation likely will be a retrofit of a North American ethanol plant, he says. This involves using the new bacteria and adding a skid for product separation. Conversion of a 100-million-gal/yr plant should run about $20 million, Gruber reckons.

Gevo also plans to offer another skid that will convert the isobutanol to materials comparable in makeup to conventional jet fuel and biodiesel. This will make economic sense at a crude oil price of $75/bbl, he says.

Meanwhile, Liao is developing modified bacteria that would work on cellulose instead of glucose, and hopes to finish constructing the strains this year.