A wider of range of plant material could be turned into biofuels thanks to a breakthrough that converts plant molecules called lignin into liquid hydrocarbons.
The reaction reliably and efficiently turns the lignin in waste products such as sawdust into the chemical precursors of ethanol and biodiesel.
In recent years, the twin threats of global warming and oil shortages have led to growth in the production of biofuels for the transportation sector.
But as the human digestive system will attest, breaking down complex plant molecules such as cellulose and lignin is a tricky business.
The biofuels industry has relied instead on starchy food crops such as corn and sugar cane to provide the feedstock for their reactions. But that puts the industry into direct competition with hungry humans, and food prices have risen as a result.
A second generation of biofuels could relieve the pressure on crop production by breaking down larger plant molecules – hundreds of millions of dollars are currently being poured into research to lower the cost of producing ethanol from cellulose.
But cellulose makes up only about a third of all plant matter. Lignin, an essential component of wood, is another important component and converting this to liquid transport fuel would increase yields.
However, lignin is a complex molecule and, with current methods, breaks down in an unpredictable way into a wide range of products, only some of which can be used in biofuels.
Now Yuan Kou at Peking University in Beijing, China, and his team have come up with a lignin breakdown reaction that more reliably produces the alkanes and alcohols needed for biofuels.
Lignin contains carbon-oxygen-carbon bonds that link together smaller hydrocarbon chains. Breaking down those C-O-C bonds is key to unlocking the smaller hydrocarbons, which can then be further treated to produce alkanes and alcohol.
But there are also C-O-C bonds within the smaller hydrocarbons which are essential for alcohol production and must be kept intact. Breaking down the C-O-C bonds between chains, while leaving those within chains undamaged, is a difficult balancing act.
In Hot Water
Kou's team used their previous experience with selectively breaking C-O-C bonds to identify hot, pressurised water – known as near-critical water – as the best solvent for the reaction.
Water becomes near-critical when heated to around 250 to 300 °C and held at high pressures of around 7000 kilopascals. Under those conditions, and in the presence of a suitable catalyst and hydrogen gas, it reliably breaks down lignin into smaller hydrocarbon units called monomers and dimers.
The researchers experimented with different catalysts and organic additives to optimise the reaction. They found that the combination of a platinum-carbon catalyst and organic additives such as dioxane delivered high yields of both monomers and dimers.
Under ideal conditions, it is theoretically possible to produce monomers and dimers in yields of 44 to 56 weight % (wt%) and 28-29 wt% respectively. Weight % is the fraction of the solution's weight that is composed of either monomers or dimers.
Impressively, the researchers' practical yields approached those theoretical ideals. They produced monomer yields of 45 wt% and dimer yields of 12 wt% – about twice what has previously been achieved.
Removing the hydrocarbons from the water solvent after the reaction is easy – simply by cooling the water again, the oily hydrocarbons automatically separate from the water.
It is then relatively simple to convert those monomers and dimers into useful products, says Ning Yan at the Ecole Polytechnique Fédérale de Lausanne, Switzerland, and a member of Kou's team.
That results in three components: alkanes with eight or nine carbon atoms suitable for gasoline, alkanes with 12 to 18 carbons for use in diesel, and methanol.
"For the first time, we have produced alkanes, the main component of gasoline and diesel, from lignin, and biomethanol becomes available," says Yan.
"A large percentage of the starting material is converted into useful products," he adds. "But this work is still in its infancy so other aspects related to economic issue will be evaluated in the near future."
John Ralph at the University of Wisconsin in Madison thinks the work is exciting. He points out that there have been previous attempts to convert lignin into liquid fuels. "That said, the yields of monomers [in the new reaction] are striking," he says.
Richard Murphy at Imperial College London, UK, is also impressed with Kou's work. "I believe that approaches such as this will go a considerable way to help us extract valuable molecules including fuels from all components of lignocellulose," he says.
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