Instead of making fuels by capturing the rays of the sun, how about we let plants store the sun’s energy for us and then turn plant matter into fuels? Biofuels such as ethanol made from corn and biodiesel made from seeds have already found a place in the energy markets, but they threaten to displace food crops, particularly in developing countries where selling biofuels abroad can be more lucrative than feeding people at home.
The numbers are daunting: meeting current oil demand would mean requisitioning huge areas of arable land. Turning food into energy, then, may not be the best approach. One answer could be to exploit other, less vital forms of biomass. The U.S. produces enough agricultural and forest residue to supply nearly a third of the annual consumption of gasoline and diesel for transportation.
Converting this low-grade biomass into fuel requires breaking down hardy molecules such as lignin and cellulose, the main building blocks of plants. Scientist already know how to do that, but the existing methods tend to be too expensive, inefficient or difficult to scale up for the enormous quantities of fuel that the economy needs.
One of the challenges of breaking down lignin—cracking open the carbon-oxygen bonds that link “aromatic,” or benzenetype, rings of carbon atoms—was recently met by John Hartwig and Alexey Sergeev,
both at the University of Illinois. They found a nickel-based catalyst able to do it. Hartwig points out that if biomass is to supply nonfossil-fuel chemical feedstocks as well as fuels, scientist will also need to extract aromatic compounds (those having a backbone of aromatic rings) from it. Lignin is the only major potential source of such aromatics in biomass. To be practical, such conversion of biomass will, moreover, need to work with mostly solid biomass and convert it into liquid fuels for easy transportation along pipelines. Liquefaction would need to happen on-site, where the plant is harvested. One of the difficulties for catalytic conversion is the extreme impurity of the raw material—classical chemical synthesis does not usually deal with messy materials such as wood. “There’s no consensus on how all this will be done in the end,” Hartwig says. What is certain is that an awful lot of any solution lies with the chemistry, especially with finding the right catalysts. “Almost every industrial reaction on a large scale has a catalyst associated” with it, Hartwig points out.
SOURCE : SCIENTIFIC AMERICAN MAGAZINE OCTOBER 2011