New metal catalysts to break down cellulose
Whenever we hear about cellulose ethanol, we mainly think of specially designed enzymes that break down the material to release the sugars contained in it. But there is also a catalytic conversion process, that has received less attention. Japanese chemists from the University of Hokkaido have put it back on the agenda. They found that a new kind of metal catalysts based on platinum and ruthenium too can break down cellulose into simple sugar alcohols in an equally or even more efficient way, marking an important step in the quest to produce green fuels from more abundant biomass resources.
'First generation' biofuels only use easily extractable and fermentable sugars or starches from crops (such as sugarcane or cassava), without using the ligno-cellulosic parts of the plant that are considered to be 'waste'. Second generation biofuels precisely focus on these 'leftovers' that are more difficult to process into sugars. Ethanol would be much greener if it could be produced from this cellulose which is found in great abundance in agricultural waste streams, such as straw, wood chips, or bagasse (a byproduct of sugar production from sugar cane). Breaking down cellulose is far more difficult, though.
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 190ºC. 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:
ethanol :: biomass :: bioenergy :: biofuels :: energy :: sustainability :: cellulose :: catalyst :: platinum ::
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, thinks 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.
More information:
Platinum Today: PGMs help biofuel conversion - October 13, 2006
Chemistry World: Catalyst cracks tough cellulose - July 13, 2006
http://www.rsc.org/chemistryworld/News/2006/July/13070601.asp
'First generation' biofuels only use easily extractable and fermentable sugars or starches from crops (such as sugarcane or cassava), without using the ligno-cellulosic parts of the plant that are considered to be 'waste'. Second generation biofuels precisely focus on these 'leftovers' that are more difficult to process into sugars. Ethanol would be much greener if it could be produced from this cellulose which is found in great abundance in agricultural waste streams, such as straw, wood chips, or bagasse (a byproduct of sugar production from sugar cane). Breaking down cellulose is far more difficult, though.
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 190ºC. 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:
ethanol :: biomass :: bioenergy :: biofuels :: energy :: sustainability :: cellulose :: catalyst :: platinum ::
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, thinks 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.
More information:
Platinum Today: PGMs help biofuel conversion - October 13, 2006
Chemistry World: Catalyst cracks tough cellulose - July 13, 2006
http://www.rsc.org/chemistryworld/News/2006/July/13070601.asp
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