Biofuel production currently involves a complex mixture of hydrophilic and hydrophobic liquids, along with one or more catalysts. Getting them all together and separating out the fuel can be a time-consuming challenge. Researchers have now used carbon nanotubes and oxidized metals to create a solid that is both hydrophilic and hydrophobic and sits between oil and alcohol layers, mediating their interactions.

Making biofuel using current methods can be a bit tedious. Recipes generally involve mixing some kind of bio-oil, often vegetable oil, with an alcohol, usually methanol, along with a catalyst such as lye. Once these have all been combined, they react to form the desired biofuel, glycerine, and some excess soap, water, and alcohol. All of these will, for the most part, separate into layers like with a vinaigrette dressing if allowed to sit for a long enough time.

The glycerine can be drained off easily enough, and most of the impurities will settle between the glycerine and biofuel, but the biofuel must be "washed" a few times to extract any errant soap particles and other impurities that are suspended in it, and boiled to remove the water. All told, the process can take between a couple of days and a week, depending on how much you're making. There are machines that will carry out the mixing and washing, but the process can't be shortened much because of the impurities that are introduced due to the use of lye as a catalyst.

Researchers set out to solve this problem by finding a catalyst that would not introduce any impurities that would be difficult to remove. They also wanted to find one that would that could stabilize an oil and water emulsion, which would help the reaction components form a stable mix, in the same way that egg yolks stabilize mayonnaise. A stabilized emulsion would significantly increase the surface area where the two substances can react—typically, this function is performed by the solid catalysts. Ideally, the newly engineered catalysts would also be reusable.

The researchers' solution involved a combination of hydrophilic and hydrophobic materials that would both emulsify the oil/water mixture by sitting at the interface of the two substances, and facilitate their reaction to form biofuels. To accomplish this, they grew hydrophobic carbon nanotubes on small pellets of hydrophilic oxidized metals that contained enough palladium catalyst to speed up the reaction.

They found this combination helped the aqueous and organic phases emulsify, and would remain at the boundary between the two substances; the palladium facilitated the hydrogenation, hydrogenolysis, and decarbonylation reactions. Hydrogenation was the dominant reaction at around 100�C, hydrogenolysis at 200�C, and decarbonylation at 250�C. Each of these reactions is useful for the conversion of different combinations of alcohols and oils, and because of the increased surface area. Thanks to the inclusion of palladium, these reactions happen at a much faster rate than when performed using lye.

Once the reactions had occurred, the authors found that all of the desired products had moved into the organic phase, or what was once just bio-oil, leaving any waste and water in the aqueous phase, where it was still bound by the catalytic nanoparticles.

To separate the catalyst and waste, they strained the liquid through a regular paper filter, which managed to catch most of the catalyst. They then passed the organic liquid through a polytetrafluoroethylene filter to catch the nanoparticles that had gotten through the paper filter, leaving them with purified biofuel.

These solid nanohybrid particles seem to be a strong candidate for fuel production, given the greater amount of precision and control they provide fuel makers and the speedier reaction times they enable. But they do still require a filtration process, an aspect of the experiment that was not extensively studied. Since reducing production time and increasing purity would be beneficial to the future of biofuel, streamlining the waste-removal step in this process will be critical. The paper also made no mention of whether their chosen nanoparticles were reusable after their initial reaction. Still, the basic principles seem solid, provided that these aspects of the catalysts can be optimized.

Science, 2009. DOI: 10.1126/science.1180769