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Biobutanol enters battle of the alcohols

02 January 2008

Recent breakthroughs have raised the tempting prospect that butanol, fermented from designer microbes, could supersede ethanol as a renewable fuel additive. The latest research comes from Jim Liao and colleagues at the University of California, Los Angeles, US, who re-engineered the Escherichia coli  bacterium to force it to produce isobutanol and a variety of other promising long-chain alcohol fuels.1  

Biobutanol enters battle of the alcohols
Edward Green, of Green Biologics, and his butanol-brewing bacteria

Butanol is far superior to ethanol as a petrol substitute. It stores more energy per litre (giving it 88 per cent of the mileage of petrol, versus ethanol's 70 per cent); it's less prone to water contamination; and it's less corrosive to pipelines than ethanol. It can also be blended into petrol at higher concentrations without having to modify vehicles.  

Since 1916, it's been known that microbes such as Clostridium acetobutylicum  can ferment sugar to produce butanol, acetone and ethanol - the 'ABE' process exploited mainly for its acetone during the first world war. 

 Yet microbial breweries were discarded by the 1980s in favour of a cheaper petrochemical route, via the reaction of carbon monoxide and hydrogen with propylene. The main difficulty was - and still is - that bacteria are poisoned by the butanol they produce once its concentration rises above about 2 per cent.  

Hijack squad 

So Liao's team has hijacked bacterial metabolism to improve butanol tolerance and yield. Their main target, branched-chain isobutanol, has a higher octane rating than straight-chain butanol, and is also a useful intermediate for fine chemicals. The researchers avoided traditional ABE fermentation, instead introducing genes expressing foreign enzymes that diverted excess 2-keto acids (intermediates in amino acid synthesis) to alcohols, and using robust E. colias a proof of principle. 'Because this strategy can readily be transferred to other organisms, it can be implemented in organisms that [unlike E. coli] degrade cellulose or fix CO2,' Liao told Chemistry World

Liao coordinates his efforts with Pasadena-based biofuels company Gevo. 'We should be in the initial stages of commercial scale biobutanol production in two to three years,' Liao said.  

Another confident biobutanol team is Oxfordshire, UK-based company Green Biologics. CEO Edward Green told Chemistry Worldthat they have developed their own thermophilic bacteria to produce high yields of butanol that can tolerate close to 4 per cent butanol concentration.  

Green's clostridia  strains can also digest hemicellulose (5-carbon sugar) feedstocks, unlike corn-fed ethanol producers. This year he plans to demonstrate the process on effluent streams from a wood pulping mill in Scandinavia and molasses from sugar producers in South Africa. 'We are looking to bolt on small plants onto existing factories to produce about 10,000 tonnes of butanol a year within 18-24 months,' he said. The company's main advantage, Green said, is that it has the expertise to modify strains which others have rejected as hard to handle. They are also investigating membrane technology to make concentrating a butanol solution much less energy-intensive than conventional distillation.  

Here come the big boys 

Looming over these endeavours are the big names: BP and DuPont, who are planning biobutanol fuel performance demonstrations in 2009. Along with British Sugar, they are building a $400 million bioethanol plant on BP's existing chemicals site at Hull, UK, which aims to produce ethanol by 2010; biobutanol may follow at the same site. DuPont are developing microbes in-house, but spokesperson Michelle Reardon explained that they had also veered away from the ABE process.  

The success of these endeavours may depend on whether biobutanol can attract the kind of subsidies that have already been liberally splashed about for corn-based ethanol in the US, a policy which has been severely criticised (see Chemistry World, October 2007, p48, and this issue, p11). But butanol has an advantage over ethanol in this regard: small companies could sell their butanol to the chemical commodities market, where it is valued at about three times the price of ethanol.  

In 2008, global consumption of butanol stood at an estimated 3.5 million tonnes, according to chemical giant BASF. It's used in solvents, plasticizers, paints, resins and as a chemical feedstock. 

John Cuzens, chief technology officer at BlueFire Ethanol, one of six cellulosic ethanol companies awarded funding from the US Department of Energy, reckons this will help biobutanol to be economically feasible in three years. 

Realistically, says Ohio-based biobutanol enthusiast David Ramey, 'butanol is in its infancy, not like ethanol with 30 years of research under its belt'. Yet many companies could pilot biobutanol units on the side of established ethanol plants, says Green. Gevo's CEO, Patrick Gruber, agrees: 'Ethanol has done a great job of seeding the market - now butanol can take full advantage of the infrastructure.'  


Richard Van Noorden


S Atsumi, T Hanai and J C Liao, Nature, 2008, 451, 86 (DOI:10.1038/nature06450)

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