Although battery-powered vehicles are making great strides, it's tough to match the energy densities found in hydrocarbons. So they'll always be an excellent choice for applications where weight or long distances are involved—think airplanes and ships—but our planet has a finite supply of easy-to-extract hydrocarbons. At some point, we'll inevitably need to look for alternatives.

Biofuel is one option with some appeal. Fossil fuels, after all, are simply the result of a bit of time and geology acting on plant matter. We should, in theory, be able to speed that process. So far, however, the easiest thing to produce has been ethanol, which isn't nearly as energy rich as the hydrocarbons in diesel and jet fuels. But a new process that mixes bacteria with traditional catalysts in a single process provides a relatively high conversion of sugar to hydrocarbons.

The process relies on a specific species of bacteria, Clostridium acetobutylicum. Given a source of sugar (which can be obtained by digesting cellulose in plants), these bacteria will produce a mixture of small carbon compounds: acetone, ethanol, and butanol. Although these chemicals can be useful in a number of contexts, they're not great as fuels. The longest of these is only four carbons, and all of them have an oxygen incorporated into their structure.

Normally, these products of metabolism will build up until they become harmful to the bacteria making them, causing the reaction to shut down. But a team of Berkeley scientists has found a solvent, glyceryl tributyrate that doesn't mix with water. It will preferentially dissolve acetone and butanol, separating them out from the water that the bacteria live in. This also leaves the ethanol behind, possibly for use as a separate biofuel.

Once separated, a simple catalyst (K 3 PO 4 and palladium) can catalyze a condensation reaction. In these reactions the butanol combines with the acetone, releasing a water molecule in the process. The reaction, however, leaves a chemical structure called a ketone behind (a carbon atom double bonded to an oxygen). This is very similar to the structure of the acetone in the first reaction, allowing a second condensation to occur.

The end result is an 11 carbon compound with a ketone in the middle, which accounts for about half of the output of the reaction (the rest is distributed among a mix of other oxygen-containing hydrocarbons). That's not quite a traditional hydrocarbon fuel, but it apparently burns very similarly to diesel fuel. And it can be fed into the existing refinery infrastructure and used as a feedstock for producing either diesel fuel or jet fuel. It's quite a bit more expensive than existing petroleum feedstocks, but their limited supply and the externalities associated with their use will inevitably tip the balance in favor of something like this.

Aside from price, there are a couple of downsides to this process. One is the use of palladium, which is quite expensive. The authors show, however, that each mole of palladium catalyst is good for producing at least 3,000 moles of reaction products before it will need to be recycled. So, this isn't as much of a cost as it appears to be. Plus, it might be possible to find a cheaper catalyst that is just as effective.

The other issue is the complex mix of hydrocarbons that result. Some of these could be potentially useful, as a lot of the materials we use in other products are made by processing petroleum. But they still have to be separated out in various ways before anything can be used—and certainly before this mix is put into a diesel engine.

Even if something better is developed before biofuels really come into their own, the approach demonstrated here is a rather interesting twist. Most efforts so far have focused exclusively on either living or metallic catalysts. By mixing the two, the authors have shown it's possible to tailor a system in which each perform the steps they're best at, and produce a result that's closer to the material we want. In the long run, it's probably better than adapting to an easy-to-make material that's less than ideal.

Nature, 2012. DOI: 10.1038/nature11594 (About DOIs).