When it comes to biofuels, ethanol is king. But it's not an ideal fuel. Among other drawbacks, it has only about two-thirds of the energy of gasoline. So scientists have been working to turn crops and crop waste into gasoline and other energy-rich hydrocarbons. Now, researchers in Wisconsin have developed a two-step process that can help convert agricultural waste into the liquid hydrocarbons found in gasoline and jet fuel.
Plants have the potential to be great sources of chemical fuels, because they are made largely of cellulose, which can be broken down into sugars and other compounds that can then be chemically converted into fuels. Researchers led by University of Wisconsin, Madison, chemist James Dumesic had previously come up with a way to accomplish this second chemical conversion step, efficiently transforming a plant-derived compound called HMF into alkenes, some of the hydrocarbons found in gasoline. But the process required other costly chemicals called ketones. Another drawback was that the process for making HMF doesn't stop: Some HMF keeps reacting, turning into two more stable compounds known as levulinic acid and formic acid, not normally used for making fuels.
Dumesic's team knew that existing catalysts could turn those acids into a small, ring-shaped compound called gamma-valerolactone (GVL). So the rather than try to stop their chemical transformation at HMF, they decided to determine whether they could devise a means for turning GVL into liquid alkenes without the need for added ketones.
The researchers report in tomorrow's Science that they succeeded with the help of two cheap, commercially available catalysts. Their process works in two steps. In the first step, the researchers flow a solution of GVL over a silica-alumina based catalyst, which breaks GVL's ring and turns it into butene, a short, linear hydrocarbon. In the next step, they expose butene at high pressure to another catalyst, which links multiple butenes together to make longer alkenes. In the end, Dumesic says that about 95% of the energy in GVL winds up in the liquid fuels.
The process could also make for a more climate-friendly gasoline. That's because turning GVL into liquid alkenes generates a pure stream of pressurized CO2 as a byproduct. In a future cellulose-based gasoline production facility, that pressurized CO2 could then either be pumped directly underground for long-term sequestration or liquefied and shipped by truck to a sequestration site, Dumesic says. CO2 would still be released when the alkenes are burned. But because plants must take more CO2 to build cellulose, the overall process would be carbon negative.
Doug Cameron, a former biofuels researcher who now serves as the Chief Science Advisor and managing director of Piper Jaffrey, an investment bank in Minneapolis, Minnesota, says he's impressed with the work because it efficiently converts GVL into fuel. However, he adds that the challenge now will be tackling the first part of the problem. "Coming up with a really robust, high-yield route [from plant material] to the GVL is the key now," Cameron says. Dumesic says he and his colleagues are now turning their attention to just that challenge. If they succeed, Cameron suggests that the technique could lead to an economical way to turn agricultural wastes into renewable fuels.