The Road to Biodiesel

Despite all the hoopla, the nation’s fleet of trucks and buses will not be pulling up to their local fast food joint and filling up with a tankful of kitchen grease any time soon. That would require retrofitting the millions of diesel-fueled vehicles currently on the road. The vegetable oils and animal fats used in cooking are composed of triglycerides, which could clog conventional diesel systems.

“For biodiesel to gain acceptance, it must have flow properties similar to ordinary diesel fuel,” says chemical engineering professor Robert Davis. Conventional liquid catalysts now used to convert heavy fats and oils to thinner biodiesel must be neutralized and disposed of at the end of the process. Davis is using funding from the U.S. Department of Energy to develop solid catalysts that would be recoverable and reusable, making biodiesel more attractive economically and environmentally.

Liquid catalysts do have one important advantage in the production of biodiesel: they can be mixed easily and thoroughly with triglycerides. To duplicate this effect, Davis’s solid catalyst will be highly porous. He is using nanodesign techniques to optimize these pores so that the triglycerides receive maximum exposure to the catalytic agents.

In his most optimistic moments, Davis doesn’t imagine that the United States could produce enough biodiesel to replace all the diesel fuel it now consumes. In his view, a more likely future for biodiesel is as a renewable fuel additive for conventional diesel engines. Biodiesel restores the lubricating qualities removed in the process of formulating low-sulfur, low-polluting fuel. But even if it is manufactured in sufficient quantities to serve as a fuel additive, processing biodiesel creates another challenge—the production of significant amounts of glycerol as a byproduct. “For every three molecules of biodiesel, we create a molecule of glycerol,” he observes. “If we start producing biodiesel on a large scale, the amount of glycerol we’re going to have on hand is going to add up quickly.”

Rather than dispose of the glycerol, Davis wants to devise a process that transforms it into useful chemicals—and here again he is breaking new ground. “Most organic chemical products use oil and natural gas as a feedstock,” Davis explains. “To make high-value chemicals from hydrocarbons, you add oxygen and increase their complexity.” Glycerol and other biomolecules are rich in oxygen to begin with, so the process of transforming them into useful feedstock requires chemists to reduce them, effectively removing some of the oxygen. In addition, petrochemical production is often done in the gas phase. Biomolecules are soluble in water, so a more appropriate approach would be to immerse them in an aqueous environment.

“To process biomolecules on an industrial scale, you need new techniques, new catalysts, and new strategies,” says Davis. With funding from the National Science Foundation, Davis is developing fundamental tools to study and manipulate the behavior of different catalysts in aqueous environments. “In essence, we’re learning how to convert sugars into the stuff of everyday life,” he says.