Kinder, faster biodiesel conversion

BROWN U. (US) — Researchers have demonstrated a streamlined way to convert waste vegetable oil into biodiesel, eliminating the need for corrosive chemicals to perform the reactions.

Chemists from Brown University were able to pull off the waste vegetable oil-to-biodiesel conversion in a single reaction vessel using environmentally friendly catalysts—and making the conversion six times faster than current methods, which are costly and inefficient.

Current methods also require the toxic chemicals sulfuric acid and either potassium hydroxide or sodium hydroxide.

“We wanted to develop an environmentally benign and technically simple way to convert waste vegetable oil into biodiesel,” says Jason Sello, assistant professor of chemistry. “The production of energy at the expense of the environment is untenable and should be avoided at all costs.”

Sello and colleague Aaron Socha report their results in the journal Organic & Biomolecular Chemistry.

Waste vegetable oil is made up of triacylglycerols, free fatty acids, and water. The conventional way to convert waste vegetable oil into biodiesel requires two separate reactions.


Aaron Socha, left, and Jason Sello devised a way to convert waste vegetable oil to biodiesel in a single reaction vessel, using environmentally friendly catalysts. (Credit: Mike Cohea/Brown University)

The first reaction turns the free fatty acids into biodiesel, but that conversion requires sulfuric acid.

The second reaction converts the triacylglycerols into biodiesel, but that conversion requires sodium hydroxide or potassium hydroxide. Sodium hydroxide/potassium hydroxide and sulfuric acid are not compatible with each other, so the reactions must be carried out in separate vessels. That makes the process less efficient.

To find a better way, Sello and Socha went looking for catalysts that would be cheap, chemically stable and of limited toxicity. They settled on the metals bismuth triflate and scandium triflate, commonly used as catalysts in preparative organic chemistry.

In addition, they performed the reactions using a microwave reactor instead of a conventional thermal heater. What they found was the new catalysts converted waste vegetable oil into biodiesel in about 20 minutes in the microwave reactor, whereas current reactions without catalysts using a conventional heater take two hours.

While their microwave method needs a higher temperature to pull off the biodiesel conversion—150 degrees Celsius versus 60 degrees Celsius under current methods—it uses less energy overall because the reaction time is much shorter.

The chemists also were able to perform the conversion in one reaction vessel, since the catalysts can promote both the reaction that converts free fatty acids into biodiesel and the reaction in which triacylgycerols are converted to biodiesel.

The team also reports that the catalysts in the free fatty acid conversion, which is the more challenging of the two reactions, could be recycled up to five times, while maintaining the capacity to promote a 97 percent reaction yield. The fact the catalysts can be recycled lowers their cost and environmental impact, the researchers said.

“While we have not yet proven the viability of our approach on an industrial scale,” Sello says, “we have identified very promising catalysts and reaction conditions that could, in principle, be used for large-scale conversion of waste vegetable oil into biodiesel in an enviornmentally sensitive manner.”

The research was funded in part by the National Science Foundation.

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