Could straw furnish the fuel of the future?

Metal catalysts can break down cellulose into simple sugar alcohols, Japanese chemists have found, marking an important step in the quest to produce green fuels from renewable resources. The catalysts could ultimately turn relatively undigestible plant materials into so-called biofuels, or important feedstock chemicals.1

The search for alternatives to fossil fuels dates back to the petrol crisis of the early 1970s, but the looming dangers of climate change combined with high petrol prices are driving a renewed interest in biofuels.

Countries and companies that have persisted in the quest since the early days are now leading the field. Brazil, for example, already runs half of its vehicle fleet on pure ethanol, made by fermenting sugar cane extracts. However, this approach relies on high-quality biomass, which is produced at a cost and could just as well serve as food for humans or livestock. Bioethanol would be much greener if it could be produced from agricultural waste materials, such as straw, wood chips, or bagasse (a byproduct of sugar production from sugar cane), a conclusion confirmed by a recent analysis of biofuel efficiency.2 But this means breaking apart the inedible plant polymer cellulose, which is much tougher than the easily digestible starch. 

Enzymes can help to chew up cellulose, and the Iogen Corporation has a demonstration plant in Ottawa, Canada, that can turn 25 tonnes of wheat straw into ethanol every week. In May 2006, Iogen made headlines by attracting the first Wall Street investment into ethanol, a $30 million cash injection from Goldman Sachs, which is earmarked to accelerate Iogen’s commercialisation program. 

But Atsush Fukuoka and Paresh Dhepe of Hokkaido University, Japan, claim to have developed two metal catalysts that could outperform enzymes. They used platinum and ruthenium, supported on silica or alumina, to convert an aqueous mixture of cellulose and hydrogen gas into glucose at about 190?C. This sugar was then reduced to the sugar alcohols sorbitol and mannitol, which were easily separated from the reaction in an overall yield of 31 per cent. Sorbitol can be used to make fuel hydrocarbons,3 while both sugar alcohols are useful feedstock compounds. It may also be possible to extract glucose directly from the reaction, which could then be fermented to produce ethanol. 

Puncturing prejudice

The use of inorganic catalysts is a new avenue to the old problem of breaking up cellulose. ’People have had the prejudice that the catalytic route does not work due to less interaction between solid cellulose and solid catalyst,’ said Fukuoka. "We have overcome this problem by increasing the [number of] acidic sites generated in situ from hydrogen gas on the catalyst."

Carlos Martin, who developed a cellulose ethanol plant at the University of Matanzas, Cuba, told Chemistry World that the catalytic process was ’very interesting’, but that ’it should be tested at a larger scale before drawing any conclusions about its suitability’.

’But provided that the process works well, it could also be applied to the conversion of other polysaccharides to sugar alcohols,’ he added, ’such as the conversion of xylan to xylitol’.

Whether the catalytic method will be able to compete in a rapidly moving market, where processes based on enzymes are already run on the tonne scale remains to be seen. But Fukuoka remains optimistic: ’I think at this moment that we need complementary use of catalytic processes, enzymatic processes, supercritical fluid methods, and so on,’ he said.

There is certainly plenty of potential for investment, with biofuels featuring prominently in the US Department of Energy’s recent roadmap for cleaner fuels. ’Cellulosic ethanol has the potential to be a major source for transportation fuel for America’s energy future’ said the under secretary for science, Raymond Orbach.The US government aims to replace 30 per cent of the current amount of transport fuel used with biofuels by the year 2030. 

Michael Gross