On the road to more energetic biofuels

US researchers have developed a way to create precursor molecules of petrol, diesel and jet fuel by combining biological fermentation with chemical catalysis. The work offers a potential new route for producing green and renewable transportation fuels of the future.

Fermentation is well established as an effective route to produce low molecular mass compounds including ethanol and other alcohols from sugars contained in biomass. However, such alcohol-based biofuels have fewer carbon atoms and, therefore, less energy than petrol and diesel and are also are not compatible with the current transport fuel infrastructure.

Now, Dean Toste and colleagues at the University of California, Berkeley, have developed an efficient tuneable process that incorporates fermentation and catalysis to convert a range of biomass feedstocks into the hydrocarbons precursors to petrol, diesel and potentially jet fuel. 'We were able to combine fermentation and chemical processes to produce a fuel from renewable sources,’ Toste says. ‘Most processes use one or the other, but we believe the combination has the potential to provide powerful and enabling technologies.'

'The products of this process are more energy rich per litre than the lower molecular weight alcohols typically available from fermentation,' he adds. 'Compared to a purely chemical approach, this process provides for near theoretical maximum carbon yields from a range of sugars without the need for hydrogen.'

The team first obtained acetone:butanol:ethanol (ABE) – a fermentation product generated by the bacterium Clostridium acetobutylicum – via a two-step extraction and distillation process. They then subjected this stream of acetone and alcohols to an aldol-type condensation. They did this using potassium phosphate in tandem with a supported palladium catalyst to promote alkylation whereby the three carbon unit of acetone is elaborated by a series of aldol reactions with the C2 and C4 alcohols to give the longer chain, higher molecular mass hydrocarbons (C7–C11).

ABE fermentation was established and scaled in the early part of the last century, but while not currently practiced in the US or the UK, it should be relatively simple to implement Toste says. 'The chemistry portion is less proven on scale, but given that it involves heterogeneous catalysts, it should be amenable to scale up.'

'While they have demonstrated the range of possible products, the biological step is still separate from the subsequent catalytic conversion,' comments Mark Keane who investigates catalysis engineering at Heriot-Watt University in Edinburgh, UK. 'Once fermentation is combined with optimised chemo-catalysis in an actual unit operation, this work will translate into real commercial benefits across the chemical and energy sector.'

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